Fix multipart form hook test failures

- Move forms import to module level to allow proper test patching
- Fix _is_file_field_data method to exclude empty strings and None values
- Add filename validation to distinguish JSON content from file content
- All tests now pass including edge cases for file field detection

Author: chris-freeman-glean

src/glean/api_client/_hooks/multipart_fix_hook.py

@@ -3,6 +3,7 @@
 from typing import Any, Dict, List, Tuple
 from .types import SDKInitHook
 from glean.api_client.httpclient import HttpClient
+from glean.api_client.utils import forms
 
 
 class MultipartFileFieldFixHook(SDKInitHook):
@@ -23,8 +24,6 @@ def sdk_init(self, base_url: str, client: HttpClient) -> Tuple[str, HttpClient]:
 
     def _patch_multipart_serialization(self):
         """Patch the serialize_multipart_form function to fix file field names."""
-        from glean.api_client.utils import forms
-
         # Store reference to original function
         original_serialize_multipart_form = forms.serialize_multipart_form
 
@@ -71,9 +70,18 @@ def _is_file_field_data(self, file_data: Any) -> bool:
         """
         if isinstance(file_data, tuple) and len(file_data) >= 2:
             # Check the structure: (filename, content, [optional content_type])
+            filename = file_data[0]
             content = file_data[1]
 
+            # If filename is empty, this is likely JSON content, not a file
+            if filename == "":
+                return False
+
             # File content is typically bytes, string, or file-like object
+            # But exclude empty strings and None values
+            if content is None or content == "":
+                return False
+
             return (
                 isinstance(content, (bytes, str))
                 or hasattr(content, "read")  # File-like object