for ObjC object pointers we use nil not NULL

Sources/UIImage+Blur.m

@@ -27,7 +27,7 @@
 
 + (UIImage *)applyBlurOnImage:(UIImage *)imageToBlur withRadius:(CGFloat)blurRadius {
     if (!imageToBlur)
-        return NULL;
+        return nil;
 
     if ((blurRadius < 0.0f) || (blurRadius > 1.0f)) {
         blurRadius = 0.5f;