refactor: simplify user attributes callback by removing null value sanitization

Author: thomson-t

ios/RNMParticle/RNMParticle.mm

@@ -164,13 +164,7 @@ + (void)load {
     MParticleUser *selectedUser = [[MParticleUser alloc] init];
     selectedUser.userId = [NSNumber numberWithLong:mpid.longLongValue];
     NSDictionary *attributes = [selectedUser userAttributes] ?: @{};
-    NSMutableDictionary *sanitizedAttributes = [[NSMutableDictionary alloc] initWithCapacity:attributes.count];
-    [attributes enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
-        if (obj != nil && obj != [NSNull null]) {
-            sanitizedAttributes[key] = obj;
-        }
-    }];
-    callback(@[[NSNull null], sanitizedAttributes]);
+    callback(@[[NSNull null], attributes]);
 }
 
 RCT_EXPORT_METHOD(setUserTag:(NSString *)mpid tag:(NSString *)tag)