set props only if defined

[pyramation]

CREATE FUNCTION ast.a_const (
      v_val jsonb DEFAULT NULL
  ) RETURNS jsonb AS $EOFCODE$
  DECLARE
      result jsonb = '{"A_Const":{}}'::jsonb;
  BEGIN
      result = ast.jsonb_set(result, '{A_Const, val}', v_val);
      RETURN result;
  END;
  $EOFCODE$ LANGUAGE plpgsql IMMUTABLE;