[ASImageNode+AnimatedImage] Fix early return when animatedImage is nil in setAnimatedImage #trivial

[flovouin]

Hey guys,

I recently tried to update to master and ran into an issue where the defaultImages I set in my ASNetworkImageNode are replaced by nil when they leave the preload range.
It turns out, #896 does not take into account the case where the animatedImage might be nil. Hence the call to _locked_setAnimatedImage no longer performs an early return like it used to, because animatedImage.playbackReady is NO.
This PR fixes this and gives me back my defaultImages, even after they've gone out of the preload range and back. :-D

This time I haven't set up a sample project like I usually do, but I made up for it by adding a new test, yay! This test shows that the defaultImage is currently incorrectly set to nil when the ASNetworkImageNode leaves the preload range.
I've also slightly improved the (very similar) existing test for the image property, because didExitPreloadState wasn't actually called (see the diff and the related comment).

Please tell me how this looks and if I can improve anything!

Cheers,

Flo

[ghost]

	1 Warning
⚠️	Any source code changes should have an entry in CHANGELOG.md or have #trivial in their title.

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[Adlai-Holler]

Awesome diff as always @flovouin!