Set animations to empty list if null in SerializedSkin.Builder

[irrld]

When builder.animations(...) is not called, it causes builder.build() to fail with the following stack trace. This PR fixes the issue by initializing the animations to empty list just like what's being done with personaPieces and tintColors

Caused by: java.lang.NullPointerException: Cannot invoke "java.util.Collection.size()" because "c" is null
	at it.unimi.dsi.fastutil.objects.ObjectArrayList.<init>(ObjectArrayList.java:135) ~[lux.jar:?]
	at org.cloudburstmc.protocol.bedrock.data.skin.SerializedSkin.of(SerializedSkin.java:101) ~[lux.jar:?]
	at org.cloudburstmc.protocol.bedrock.data.skin.SerializedSkin$Builder.build(SerializedSkin.java:322) ~[lux.jar:?]