Fix nil exeption for pull reviews API #15104

[a1012112796]

same with master branch

gitea/modules/convert/user.go

Lines 16 to 18 in 8567cba

	if user == nil {
	return nil
	}

fix #15104

[techknowlogick]

@a1012112796 please send backport :)

[a1012112796]

@techknowlogick not need backport, this change target to release/v1.13, and the other versions has been changed by #12856. Thanks

[techknowlogick]

Thanks :)