fix: treat empty config_profile_id as no selection

Frontend was sending empty string for config_profile_id when no profile
was selected, causing 'not compatible' validation error. Backend now
treats any falsy value (None, empty string) as 'no profile selected'.

apps/api/src/api/tool_instances.py

@@ -475,7 +475,7 @@ async def _validate_config_profile(
     Raises:
         HTTPException: If profile is not found, not owned, or incompatible.
     """
-    if profile_id is None:
+    if not profile_id:
         return None
 
     try: