Align if conditions with Ruby Liquid for missing values

[ahmedadamio]

Ruby Liquid treats missing variables in condition expressions as nil, and still applies filters on them. liquid-rust did not fully match that behavior.

Before this PR in liquid-rust:
{% if missing | upcase %} skipped filter application and evaluated as falsy
{% if missing == nil %} and {% if missing | upcase == "" %} could raise at runtime
unknown filters inside conditions could degrade into a generic parse error
This PR brings if / unless behavior closer to the Ruby Liquid implementation by:

treating missing values in conditions as nil
applying filters to missing values in condition paths
making binary comparisons use the same nil-tolerant evaluation as existence checks
preserving proper Unknown filter errors in conditions
Existing targeted tests continued to pass as expected. I re-ran:

cargo test -p liquid-lib if_block
cargo test -p liquid-lib assign
cargo fmt --all --check
Added / updated test coverage:

plain_existence_check_with_missing_value_is_falsy
filter_chain_existence_check_with_missing_value_applies_filters
missing_value_comparison_treats_missing_as_nil
missing_value_comparison_with_filters_applies_filters
unknown_filter_in_condition_reports_filter_error
This improves Shopify/Ruby Liquid compatibility and makes condition behavior more predictable for template authors.

• [x ] Tests created for any new feature or regression tests for bugfixes.