fix(andoid): crash when setting font on tab-item with no image

nativescript-core/ui/bottom-navigation/bottom-navigation.android.ts

@@ -614,6 +614,9 @@ export class BottomNavigation extends TabNavigationBase {
 
     private getIcon(tabStripItem: TabStripItem): android.graphics.drawable.BitmapDrawable {
         const iconSource = tabStripItem.image && tabStripItem.image.src;
+        if (!iconSource) {
+            return null;
+        }
 
         let is: ImageSource;
         if (isFontIconURI(iconSource)) {