Only set reftitle if there was one in the first place

Co-authored-by: Stan Ulbrych <stan@python.org>

Author: hugovk

pep_sphinx_extensions/pep_processor/transforms/pep_references.py

@@ -55,4 +55,5 @@ def apply(self) -> None:
             if node.get("internal", True):
                 continue
             reftitle = node.get("reftitle", "")
-            node["reftitle"] = self.version_pattern.sub(r"(in \1)", reftitle)
+            if reftitle:
+                node["reftitle"] = self.version_pattern.sub(r"(in \1)", reftitle)