Check for 'url' key existence before using

[Ruxton]

At this moment, there is a situation where cite only has 'name' key and nothing else.

Fixes:

Undefined index: url in wp-content/plugins/indieweb-post-kinds/templates/reply-metabox.php on line 20

[dshanske]

@Ruxton the normalize_cite function was designed to always ensure there was a URL. I'd rather fix that.

[Ruxton]

@dshanske I saw that as normalize_cite gets called right after this block, but didn't push it through that function it because I notice we're potentially re-assigning the cite inside this block. Seemed a bit slow to normalize it only to potentially re-assign it and normalize it again.

[dshanske]

Good point. I just introduced it to try and solve the problem of a whole bunch of isset checks. Let me think on this one.