fix: add safety check for empty model_suffix in PostgreSQL vector storage

Why this change is needed:
Prevent potential errors when embedding_func does not have model_name set,
which could cause table naming issues in PostgreSQL.

How it solves it:
- Check if model_suffix is not empty before appending to table name
- Fall back to base table name with a warning if model_suffix is unavailable
- Log clear warning message to alert users about missing model isolation

Impact:
- Prevents crashes when model_name is not configured
- Provides clear feedback to users about configuration issues
- Maintains backward compatibility with configs that don't set model_name

Testing:
Existing PostgreSQL tests validate the happy path. This adds defensive handling
for edge cases.

lightrag/kg/postgres_impl.py

@@ -2231,7 +2231,16 @@ class PGVectorStorage(BaseVectorStorage):
             raise ValueError(f"Unknown namespace: {self.namespace}")
 
         # New table name (with suffix)
-        self.table_name = f"{base_table}_{self.model_suffix}"
+        # Ensure model_suffix is not empty before appending
+        if self.model_suffix:
+            self.table_name = f"{base_table}_{self.model_suffix}"
+        else:
+            # Fallback: use base table name if model_suffix is unavailable
+            self.table_name = base_table
+            logger.warning(
+                f"Model suffix unavailable, using base table name '{base_table}'. "
+                f"Ensure embedding_func has model_name for proper model isolation."
+            )
 
         # Legacy table name (without suffix, for migration)
         self.legacy_table_name = base_table