refactor: hoist Optional None check before Pydantic branch

Author: yuvrajangadsingh

src/google/adk/tools/function_tool.py

@@ -138,12 +138,14 @@ def _preprocess_args(
           target_type = non_none_types[0]
           is_optional = len(union_args) != len(non_none_types)
 
+      # Skip None values only for Optional params
+      if args[param_name] is None and is_optional:
+        continue
+
       # Pydantic models: keep existing graceful-failure behavior
       if inspect.isclass(target_type) and issubclass(
           target_type, pydantic.BaseModel
       ):
-        if args[param_name] is None and is_optional:
-          continue
         if not isinstance(args[param_name], target_type):
           try:
             converted_args[param_name] = target_type.model_validate(
@@ -158,10 +160,6 @@ def _preprocess_args(
             )
         continue
 
-      # Skip None values only for Optional params
-      if args[param_name] is None and is_optional:
-        continue
-
       # Validate and coerce all other annotated types using TypeAdapter.
       # This handles primitives (int, float, str, bool), enums, and
       # container types (list[int], dict[str, float], etc.).