allow for nullable style layer predicate

[incanus]

As with Core Data's NSFetchRequest, we should allow removal of a style layer predicate by setting it to nil. That other API also converts the TRUE (always on) predicate to nil upon roundtripping. Otherwise, there's not an obvious way to remove the predicate (Studio analog: filters) from a layer.

[1ec5]

The predicate property was declared as nullable but setting it to nil didn’t work. This issue was reported again in #7704 and fixed in #7705, which made it into v3.4.0.