fix: Fix cases where execution_result_delimiters have None type element due to code_execution_result.output is None

PiperOrigin-RevId: 861933883

Author: google-genai-bot

src/google/adk/code_executors/code_execution_utils.py

@@ -251,9 +251,13 @@ def convert_code_execution_parts(
     # Skip if the Content has multiple parts, which means the Content is
     # likely generated by the model.
     elif len(content.parts) == 1 and content.parts[-1].code_execution_result:
-      content.parts[-1] = types.Part(
-          text=execution_result_delimiters[0]
-          + content.parts[-1].code_execution_result.output
-          + execution_result_delimiters[1]
-      )
+      output = content.parts[-1].code_execution_result.output
+      if output is not None:
+        content.parts[-1] = types.Part(
+            text=execution_result_delimiters[0]
+            + output
+            + execution_result_delimiters[1]
+        )
+      else:
+        content.parts[-1] = types.Part(text='')
       content.role = 'user'