refactor: add explicit else branch in with_warn_for_invalid_lines

Makes the strict/non-strict code paths explicitly mutually exclusive
for better readability.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ahsansheraz-bonial

src/dotenv/main.py

@@ -40,10 +40,11 @@ def with_warn_for_invalid_lines(
                     "python-dotenv could not parse statement starting at line %s"
                     % mapping.original.line,
                 )
-            logger.warning(
-                "python-dotenv could not parse statement starting at line %s",
-                mapping.original.line,
-            )
+            else:
+                logger.warning(
+                    "python-dotenv could not parse statement starting at line %s",
+                    mapping.original.line,
+                )
         yield mapping