Fix regression when marking notifications as read

[Fs00]

74e0112 introduced a regression that caused notifications not being marked as read after opening them in most cases:

gh4a/app/src/main/java/com/gh4a/fragment/NotificationListFragment.java

Lines 143 to 146 in 74e0112

	LinkParser.ParseResult result = LinkParser.parseUri(getActivity(), uri, initialComment);
	if (result != null && result.intent != null) {
	intent = result.intent;
	} else {

When the above if condition is true, the notification is not marked as read.

This PR proposes a simpler fix for #1208 since the problem with "unsupported" notifications (like discussion ones) is that they don't have a URL, because if they had it, the URL would be opened in a browser/custom tab instead of doing nothing.

I've tested this fix with a PR notification and a discussion notification and it behaved as expected.

[maniac103]

That's what one gets for last minute fixes sigh
Thanks!