fix: make agent_graph_id non-nullable with empty default

The StoreAgent view defines agentGraphId as String (non-nullable).
Using empty string default for backward compatibility - empty strings
are falsy and filtered out in the agent generation code.

autogpt_platform/backend/backend/api/features/store/model.py

@@ -38,7 +38,7 @@ class StoreAgent(pydantic.BaseModel):
     description: str
     runs: int
     rating: float
-    agent_graph_id: str | None = None
+    agent_graph_id: str = ""
 
 
 class StoreAgentsResponse(pydantic.BaseModel):