fix: Prevent Integration Attribute Override with NSNull values (#25)

* fix: Prevent Integration Attribute Override with NSNull values

Author: BrandonStalnaker

mParticle-Adobe/MPIAdobe.m

@@ -36,8 +36,6 @@
 
 static NSString *const errorResponseKey = @"error_msg";
 
-static NSString *const invalidMarketingCloudId = @"<null>";
-
 static NSString *const errorDomain = @"mParticle-Adobe";
 static NSString *const serverErrorDomain = @"mParticle-Adobe Server Response";
 
@@ -172,13 +170,9 @@ - (void)sendRequestWithMarketingCloudId:(NSString *)marketingCloudId advertiserI
             return callbackWithCode(MPIAdobeErrorCodeServerError, @"Server returned an error", error);
         }
 
-        NSString *marketingCloudId = dictionary[marketingCloudIdKey];
-        if ([marketingCloudId isEqualToString:invalidMarketingCloudId]) {
-            marketingCloudId = nil;
-        }
-        
-        NSString *region = [NSString stringWithFormat:@"%@", dictionary[regionKey]];
-        NSString *blob = dictionary[blobKey];
+        NSString *marketingCloudId = [dictionary[marketingCloudIdKey] isKindOfClass:[NSString class]] ? dictionary[marketingCloudIdKey] : nil;
+        NSString *region = [dictionary[regionKey] isKindOfClass:[NSString class]] ? dictionary[regionKey] : nil;
+        NSString *blob = [dictionary[blobKey] isKindOfClass:[NSString class]] ? dictionary[blobKey] : nil;
 
         weakSelf.region = region;
         weakSelf.blob = blob;