Mark _CDB_Table_Exists() as PARALLEL UNSAFE

As pointed out by Algunenano, PL/pgSQL function which establishes an
EXCEPTION block to catch errors must be qualified with it.

scripts-available/CDB_Helper.sql

@@ -175,4 +175,4 @@ EXCEPTION
     WHEN invalid_schema_name OR undefined_table THEN
         RETURN false;
 END;
-$$ LANGUAGE PLPGSQL VOLATILE PARALLEL SAFE;
+$$ LANGUAGE PLPGSQL VOLATILE PARALLEL UNSAFE;