Infra: Make PEP abstract extration more robust (#4356)

This eliminates an AttributeError during PEP editing,
if the PEP being edited has an Abstract or Introduction
section but with no interior content (i.e. paragraphs)
yet.

In practice this should have no effect on actual
RSS feeds, since merged PEPs should always have a proper
abstract. However, this prevents a confusing error
from bubbling up during local editing.

See #4355.

Signed-off-by: William Woodruff <william@yossarian.net>
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>

Author: woodruffw

pep_sphinx_extensions/generate_rss.py

@@ -60,8 +60,10 @@ def pep_abstract(document: nodes.document) -> str:
             continue
 
         if title_node.astext() == "Abstract":
-            return node.next_node(nodes.paragraph).astext().strip().replace("\n", " ")
-        elif title_node.astext() == "Introduction":
+            if (para_node := node.next_node(nodes.paragraph)) is not None:
+                return para_node.astext().strip().replace("\n", " ")
+            return ""
+        if title_node.astext() == "Introduction":
             introduction = node.next_node(nodes.paragraph).astext().strip().replace("\n", " ")
 
     return introduction