[macos] Avoid nilling out style URL on load

-awakeFromNib nils out the styleURL property to ensure a style is always present on load. However, if the style URL is specified in the Style URL inspectable, it is set somewhere between -initWithCoder: and -awakeFromNib; nilling out the styleURL property in -awakeFromNib thus resets the style to the default. This change guards against nilling out a non-nil style URL, ensuring that -setStyleURL: is called once and only once on launch.

platform/macos/src/MGLMapView.mm

@@ -231,7 +231,11 @@ public:
 - (void)awakeFromNib {
     [super awakeFromNib];
 
-    self.styleURL = nil;
+    // If the Style URL inspectable was not set, make sure to go through
+    // -setStyleURL: to load the default style.
+    if (_mbglMap->getStyleURL().empty()) {
+        self.styleURL = nil;
+    }
 }
 
 + (NSArray *)restorableStateKeyPaths {