Fix wrong "void value expression" error

* parse.y (value_expr_check): `then` or `else` only `if` is not a
  void value expression, as the counterpart is evaluated as `nil`.
  [Bug #15932]

parse.y

@@ -10552,12 +10552,10 @@ value_expr_check(struct parser_params *p, NODE *node)
 	  case NODE_IF:
 	  case NODE_UNLESS:
 	    if (!node->nd_body) {
-		node = node->nd_else;
-		break;
+		return NULL;
 	    }
 	    else if (!node->nd_else) {
-		node = node->nd_body;
-		break;
+		return NULL;
 	    }
 	    vn = value_expr_check(p, node->nd_body);
 	    if (!vn) return NULL;

test/ruby/test_syntax.rb

@@ -1392,6 +1392,8 @@ eom
     mesg = /void value expression/
     assert_syntax_error("tap {a = (true ? next : break)}", mesg)
     assert_valid_syntax("tap {a = (true ? true : break)}")
+    assert_valid_syntax("tap {a = (break if false)}")
+    assert_valid_syntax("tap {a = (break unless true)}")
   end
 
   private