Address review comments.

Author: rosalyntan

appcheck/firebase-appcheck-playintegrity/src/main/java/com/google/firebase/appcheck/playintegrity/internal/GeneratePlayIntegrityChallengeRequest.java

@@ -15,7 +15,6 @@
 package com.google.firebase.appcheck.playintegrity.internal;
 
 import androidx.annotation.NonNull;
-import org.json.JSONException;
 import org.json.JSONObject;
 
 /**
@@ -27,9 +26,10 @@ public class GeneratePlayIntegrityChallengeRequest {
   public GeneratePlayIntegrityChallengeRequest() {}
 
   @NonNull
-  public String toJsonString() throws JSONException {
+  public String toJsonString() {
     JSONObject jsonObject = new JSONObject();
 
+    // GeneratePlayIntegrityChallenge takes an empty POST body since the app ID is in the URL.
     return jsonObject.toString();
   }
 }

appcheck/firebase-appcheck-playintegrity/src/main/java/com/google/firebase/appcheck/playintegrity/internal/GeneratePlayIntegrityChallengeResponse.java

@@ -15,7 +15,6 @@
 package com.google.firebase.appcheck.playintegrity.internal;
 
 import static com.google.android.gms.common.internal.Preconditions.checkNotNull;
-import static com.google.android.gms.common.util.Strings.emptyToNull;
 
 import androidx.annotation.NonNull;
 import androidx.annotation.VisibleForTesting;
@@ -38,8 +37,8 @@ public class GeneratePlayIntegrityChallengeResponse {
   public static GeneratePlayIntegrityChallengeResponse fromJsonString(@NonNull String jsonString)
       throws JSONException {
     JSONObject jsonObject = new JSONObject(jsonString);
-    String challenge = emptyToNull(jsonObject.optString(CHALLENGE_KEY));
-    String timeToLive = emptyToNull(jsonObject.optString(TIME_TO_LIVE_KEY));
+    String challenge = jsonObject.optString(CHALLENGE_KEY, null);
+    String timeToLive = jsonObject.optString(TIME_TO_LIVE_KEY, null);
     return new GeneratePlayIntegrityChallengeResponse(challenge, timeToLive);
   }