[osx] Fixed static analyzer warnings

include/mbgl/osx/MGLMapView.h

@@ -442,7 +442,7 @@ IB_DESIGNABLE
 /** Deselects an annotation and hides its callout popover.
     
     @param annotation The annotation object to deselect. */
-- (void)deselectAnnotation:(id <MGLAnnotation>)annotation;
+- (void)deselectAnnotation:(nullable id <MGLAnnotation>)annotation;
 
 /** A common view controller for managing a callout popover’s content view.
     

platform/osx/src/MGLMapView.mm

@@ -1929,7 +1929,7 @@ public:
 
 - (void)popoverDidShow:(__unused NSNotification *)notification {
     id <MGLAnnotation> annotation = self.selectedAnnotation;
-    if ([self.delegate respondsToSelector:@selector(mapView:didSelectAnnotation:)]) {
+    if (annotation && [self.delegate respondsToSelector:@selector(mapView:didSelectAnnotation:)]) {
         [self.delegate mapView:self didSelectAnnotation:annotation];
     }
 }