fix(sqlalchemy): satisfy mypy union-attr check with runtime guard instead of assert

Author: SyedShahmeerAli12

integrations/sqlalchemy/src/haystack_integrations/components/retrievers/sqlalchemy/sqlalchemy_table_retriever.py

@@ -168,6 +168,10 @@ def run(self, query: str) -> dict[str, Any]:
         if not self._warmed_up:
             self.warm_up()
 
+        if self._engine is None:  # pragma: no cover
+            msg = "Engine is not initialized."
+            raise RuntimeError(msg)
+
         try:
             with self._engine.connect() as conn:
                 result = conn.execute(text(query))