Fix FIAM test on device crash (#7328)

* Revert to previous nil check on experiment JSON

* Remove assert in FIRIAMDisplayExecutor

* Update CHANGELOG

* Change nullability of imageData in displayMessageWithDefinition:

* Keep nullability with card initializers, put nil message return back if imageData is nil

* Revert back to explicit non-null

* Actually shouldn't be nonnull due to other message types w/ optional imageData 😅

Author: christibbs

FirebaseInAppMessaging/CHANGELOG.md

@@ -1,3 +1,6 @@
+# 2021-1 -- v7.5.0
+- [fixed] Fixed failed assertion causing app to crash during test on device flow (#7299).
+
 # 2020-12 -- v7.3.0
 - [fixed] Fixed default display bug in apps that don't use `UISceneDelegate` (#6803).
 

FirebaseInAppMessaging/Sources/Flows/FIRIAMDisplayExecutor.m

@@ -423,7 +423,7 @@ - (void)checkAndDisplayNextContextualMessageForAnalyticsEvent:(NSString *)eventN
 
 - (FIRInAppMessagingCardDisplay *)
     cardDisplayMessageWithMessageDefinition:(FIRIAMMessageDefinition *)definition
-                          portraitImageData:(nonnull FIRInAppMessagingImageData *)portraitImageData
+                          portraitImageData:(FIRInAppMessagingImageData *)portraitImageData
                          landscapeImageData:
                              (nullable FIRInAppMessagingImageData *)landscapeImageData
                                 triggerType:(FIRInAppMessagingDisplayTriggerType)triggerType {
@@ -573,9 +573,8 @@ - (void)checkAndDisplayNextContextualMessageForAnalyticsEvent:(NSString *)eventN
                             triggerType:(FIRInAppMessagingDisplayTriggerType)triggerType {
   switch (definition.renderData.renderingEffectSettings.viewMode) {
     case FIRIAMRenderAsCardView:
-      // Image data should never nil for a valid card message.
       if (imageData == nil) {
-        NSAssert(NO, @"Image data should never nil for a valid card message.");
+        // Image data should never nil for a valid card message.
         return nil;
       }
       return [self cardDisplayMessageWithMessageDefinition:definition