fix: Null-safe EPG access when editing channel

[Warbs816]

Summary

• Fixes Unable to edit a channel (v0.10.27-dev) #887 — editing a channel crashes with "Attempt to read property 'name' on null" when the EPG channel's parent EPG record is missing
• Adds null-safe operator (?->) to getOptionLabelFromRecordUsing callback in ChannelResource.php:1310, consistent with the existing guard on line 1328

Test plan

• Edit a channel whose EPG channel has a deleted/missing parent EPG — should no longer error
• Edit a channel with a valid EPG channel — label should still display correctly

[sparkison]

This works, but the underlying problem is that the EPG relation wasn't being loaded, so the name was always empty. I've got a fix in place, closing this down 😉