Fix valid response check. (#36461)

We should not be comparing strings by pointer value.

Author: bzbarsky-apple

src/darwin/Framework/CHIP/MTRDeviceDataValidation.mm

@@ -207,7 +207,7 @@ BOOL MTRInvokeResponseIsWellFormed(NSArray<MTRDeviceResponseValueDictionary> * r
 
     // Now we know data is a dictionary (in fact a data-value).  The only thing
     // we promise about it is that it has type MTRStructureValueType.
-    if (data[MTRTypeKey] != MTRStructureValueType) {
+    if (![MTRStructureValueType isEqual:data[MTRTypeKey]]) {
         MTR_LOG_ERROR("Invoke response data is not of structure type: %@", data);
         return NO;
     }

src/darwin/Framework/CHIPTests/MTRDeviceTests.m

@@ -5580,6 +5580,19 @@ - (void)test044_InvokeResponseWellFormedness
             ],
             @"valid" : @(YES),
         },
+        @{
+            @"input" : @[
+                @{
+                    @"commandPath" : [MTRCommandPath commandPathWithEndpointID:@(0) clusterID:@(6) commandID:@(0)],
+                    @"data" : @ {
+                        @"type" : @"Structure",
+                        @"value" : @[], // Empty structure, valid
+                    },
+                },
+            ],
+            // Same as the previous test, but not using the SDK string constants
+            @"valid" : @(YES),
+        },
         @{
             @"input" : @[
                 @{