Allow filters to be cleared

We need to allow empty setting empty filters too. So that a filter can be cleared.
Consider this:
```
<SymbolLayer filter={this.state.filter} ...>
```
Where filter changes from `nil` to `non-nil` then to `nil` again. The removed `if` statement prevents resetting the filter to nil.

react-native-mapbox-gl/maps@3b03f64#diff-79220b28bb9c8c50280ea315ef9a4f9cR81

ios/RCTMGL/RCTMGLLayer.m

@@ -78,9 +78,7 @@ - (void)setFilter:(NSArray *)filter
 
     if (_styleLayer != nil) {
         NSPredicate *predicate = [self buildFilters];
-        if (predicate) {
-            [self updateFilter:predicate];
-        }
+        [self updateFilter:predicate];
     }
 }