add null check for response

Author: yvonnep165

src/main/java/com/google/firebase/appcheck/internal/AppCheckTokenVerifier.java

@@ -290,6 +290,9 @@ private boolean verifyOneTimeToken(String token)
       httpResponse = httpRequest.execute();
 
       GenericJson response = httpResponse.parseAs(GenericJson.class);
+      if (response == null) {
+        return false;
+      }
       Boolean alreadyConsumed = (Boolean) response.get("alreadyConsumed");
       if (alreadyConsumed == null) {
         alreadyConsumed = (Boolean) response.get("already_consumed");

src/test/java/com/google/firebase/appcheck/internal/AppCheckTokenVerifierTest.java

@@ -209,6 +209,36 @@ public void testVerifyToken_WithConsumeOption_CallsBackend() throws Exception {
     assertTrue(response.isAlreadyConsumed().get());
   }
 
+  @Test
+  public void testVerifyToken_WithConsumeOption_EmptyBackendResponse_DoesNotThrow()
+      throws Exception {
+    when(mockJwtProcessor.process(any(SignedJWT.class), any())).thenReturn(claims);
+
+    MockLowLevelHttpResponse mockResponse = new MockLowLevelHttpResponse();
+    mockResponse.setStatusCode(204);
+    mockResponse.setContent("");
+
+    MockHttpTransport transport =
+        new MockHttpTransport.Builder().setLowLevelHttpResponse(mockResponse).build();
+
+    HttpRequestFactory mockRequestFactory = transport.createRequestFactory();
+    JsonFactory jsonFactory = ApiClientUtils.getDefaultJsonFactory();
+
+    FirebaseApp app = FirebaseApp.getInstance();
+    AppCheckTokenVerifier customVerifier =
+        new AppCheckTokenVerifier(app, mockRequestFactory, jsonFactory, mockJwtProcessor);
+
+    String token = createToken(header, claims);
+    VerifyAppCheckTokenOptions options =
+        VerifyAppCheckTokenOptions.builder().setConsume(true).build();
+    VerifyAppCheckTokenResponse response = customVerifier.verifyToken(token, options);
+
+    assertNotNull(response);
+    assertEquals(APP_ID, response.getAppId());
+    assertTrue(response.isAlreadyConsumed().isPresent());
+    assertFalse(response.isAlreadyConsumed().get());
+  }
+
   @Test
   public void testVerifyToken_NullOrEmptyToken_ThrowsException() {
     IllegalArgumentException ex1 =