fix: handle None or empty contents in Gemini token counter (#17020)

Adds null/empty check before processing contents in GoogleAIStudioTokenCounter
to prevent errors when contents is None or empty.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <noreply@anthropic.com>

litellm/llms/gemini/count_tokens/handler.py

@@ -30,6 +30,10 @@ class GoogleAIStudioTokenCounter:
 
         from google.genai.types import FunctionResponse
 
+        # Handle None or empty contents
+        if not contents:
+            return contents
+
         cleaned_contents = copy.deepcopy(contents)
 
         for content in cleaned_contents: