Implement default_if_none builtin filter

[UnknownPlatypus]

Part of #1

Very similar to DefaultFilter

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[github-actions]

Django test suite conformance

ℹ️ Test pass rate changed from 33.26% to 33.58%.

--- main_test_output	2025-11-19 20:24:15.535671392 +0000
+++ branch_test_output	2025-11-19 20:24:35.995662051 +0000
@@ -1,5 +1,5 @@
-Django test suite passing: 33.26%
-1187 ERROR / 97 FAIL / 640 OK
+Django test suite passing: 33.58%
+1177 ERROR / 101 FAIL / 646 OK
 # 16770 -- The template system doesn't wrap exceptions, but annotates ... ERROR
 # 16770 -- The template system doesn't wrap exceptions, but annotates ... ERROR
 #10043: widthratio should allow max_width to be a variable ... ERROR
@@ -95,8 +95,8 @@
 Errors raised while compiling nodes should include the token ... ERROR
 Errors raised while compiling nodes should include the token ... ok
 Escape requirement survives lookup. ... ok
-Escaped backslash in argument ... ERROR
-Escaped backslash using known escape char ... ERROR
+Escaped backslash in argument ... ok
+Escaped backslash using known escape char ... ok
 Escaping inside blocktranslate and translate works as if it was ... ERROR
 Expression starting and ending with a quote ... ERROR
 Extending should continue even if two loaders return the same ... ERROR
@@ -134,14 +134,14 @@
 Iterating over strings outputs safe characters. ... ok
 Let's just make sure setup runs cases in the right order. ... ok
 List-index syntax allows a template to access a certain item of a ... ok
-Literal string arguments to filters, if used in the result, are safe. ... ERROR
-Literal string as argument is always "safe" from auto-escaping. ... ERROR
+Literal string arguments to filters, if used in the result, are safe. ... FAIL
+Literal string as argument is always "safe" from auto-escaping. ... FAIL
 Literal strings are safe. ... ERROR
 Logically the same as above, just written with explicit ifchanged ... ERROR
 Mixed named and unnamed endblocks ... ERROR
 More than one replacement variable is allowed in a template ... ok
 Multiple levels of attribute access are allowed. ... ok
-Nonexistent args ... ERROR
+Nonexistent args ... ok
 Numbers as filter arguments should work ... ERROR
 Optimized RequestContext construction (#7116). ... ok
 Plain text should go through the template parser untouched. ... ok
@@ -212,7 +212,7 @@
 Two-level with no redefinitions on second level ... ERROR
 Two-level with one block defined, one block not defined ... ERROR
 Using filters with the {% translate %} tag (#5972). ... ERROR
-Variable as argument ... ERROR
+Variable as argument ... ok
 Variable names that aren't resolved as literals. ... ok
 Variables should be replaced with their value in the current ... ok
 When a TemplateDoesNotExist instance is cached, the cached instance ... ERROR
@@ -761,9 +761,9 @@
 test_if_tag_and06 (template_tests.syntax_tests.test_if.IfTagTests.test_if_tag_and06) ... ok
 test_if_tag_and07 (template_tests.syntax_tests.test_if.IfTagTests.test_if_tag_and07) ... ok
 test_if_tag_and08 (template_tests.syntax_tests.test_if.IfTagTests.test_if_tag_and08) ... ok
-test_if_tag_badarg02 (template_tests.syntax_tests.test_if.IfTagTests.test_if_tag_badarg02) ... ERROR
-test_if_tag_badarg03 (template_tests.syntax_tests.test_if.IfTagTests.test_if_tag_badarg03) ... ERROR
-test_if_tag_badarg04 (template_tests.syntax_tests.test_if.IfTagTests.test_if_tag_badarg04) ... ERROR
+test_if_tag_badarg02 (template_tests.syntax_tests.test_if.IfTagTests.test_if_tag_badarg02) ... ok
+test_if_tag_badarg03 (template_tests.syntax_tests.test_if.IfTagTests.test_if_tag_badarg03) ... FAIL
+test_if_tag_badarg04 (template_tests.syntax_tests.test_if.IfTagTests.test_if_tag_badarg04) ... ok
 test_if_tag_eq01 (template_tests.syntax_tests.test_if.IfTagTests.test_if_tag_eq01) ... ok
 test_if_tag_eq02 (template_tests.syntax_tests.test_if.IfTagTests.test_if_tag_eq02) ... ok
 test_if_tag_eq03 (template_tests.syntax_tests.test_if.IfTagTests.test_if_tag_eq03) ... ok
@@ -926,7 +926,7 @@
 test_invalid_template_name_raises_template_does_not_exist (template_tests.test_partials.PartialTagsTests.test_invalid_template_name_raises_template_does_not_exist) (template_name=None) ... ERROR
 test_invalid_value (template_tests.filter_tests.test_yesno.FunctionTests.test_invalid_value) ... ok
 test_invalidstr01 (template_tests.syntax_tests.test_invalid_string.InvalidStringTests.test_invalidstr01) ... FAIL
-test_invalidstr02 (template_tests.syntax_tests.test_invalid_string.InvalidStringTests.test_invalidstr02) ... ERROR
+test_invalidstr02 (template_tests.syntax_tests.test_invalid_string.InvalidStringTests.test_invalidstr02) ... FAIL
 test_invalidstr03 (template_tests.syntax_tests.test_invalid_string.InvalidStringTests.test_invalidstr03) ... ok
 test_invalidstr04 (template_tests.syntax_tests.test_invalid_string.InvalidStringTests.test_invalidstr04) ... ok
 test_invalidstr04_2 (template_tests.syntax_tests.test_invalid_string.InvalidStringTests.test_invalidstr04_2) ... ok

[LilyFirefly]

This is great!

I think there's a bit more testing we can add: