fix: resolve pyright issue with openapi schema return type

Signed-off-by: Luca Muscariello <muscariello@ieee.org>

Author: muscariello

src/a2a/server/apps/jsonrpc/fastapi_app.py

@@ -58,7 +58,8 @@ def openapi(self) -> dict[str, Any]:
                 self.openapi_schema = json.loads(
                     schema_file.read_text(encoding='utf-8')
                 )
-                return self.openapi_schema
+                if self.openapi_schema:
+                    return self.openapi_schema
         except Exception:  # noqa: BLE001
             logger.warning(
                 "Could not load 'a2a.json' from 'a2a.types'. Falling back to auto-generation."