Make GetUISummary_UnitStatusIcons() skip empty status icons

[Iridar]

Fixes #1120

This will fix the bug where status icons would not display sometimes.

[Iridar]

I've tested and can confirm the fix works. My test was:

1. Use current Highlander
2. Go to TQL, spawn a few Advent Troopers.
3. Throw a Freeze Grenade at an Advent Trooper. Freeze grenade doesn't have a status icon set up, so it won't be displayed, but vanilla logic would try to display it anyway.
4. Launch a Napalm Rocket from my Rocket Launchers mod, which does have a status icon set up.
5. The status icon doesn't show up on the frozen enemy, but shows up on other enemies, since they don't have other display-able persistent effects applied to them before the napalm effect.

Then I applied the suggested fix in Issue #1120 and tested again, this time the Napalm effect status icon showed up for both frozen and unfrozen enemies.

[Iridar]

I didn't do a HLDocs\ref-bugfixes entry for this one, since there's no room around and I wasn't sure how to handle it.

[Xymanek]

Please add HL-Docs: ref:Bugfixes

since there's no room around and I wasn't sure how to handle it

I would put it above the if

[Iridar]

Doneso.