Allowing a Java object field to be null in a new JNI helper method.

Java objects in the API should be allowed to be null in some cases.
Specifically, a null value for maxBitrateBps in RtpParameters.java
has a specific meaning and doesn't imply an error has occurred.

NOTRY=True

Review URL: https://codereview.webrtc.org/1853523002

Cr-Commit-Position: refs/heads/master@{#12221}
diff --git a/webrtc/api/java/jni/jni_helpers.cc b/webrtc/api/java/jni/jni_helpers.cc
index f42abc7..052dcf9 100644
--- a/webrtc/api/java/jni/jni_helpers.cc
+++ b/webrtc/api/java/jni/jni_helpers.cc
@@ -176,7 +176,13 @@
 jobject GetObjectField(JNIEnv* jni, jobject object, jfieldID id) {
   jobject o = jni->GetObjectField(object, id);
   CHECK_EXCEPTION(jni) << "error during GetObjectField";
-  RTC_CHECK(o) << "GetObjectField returned NULL";
+  RTC_CHECK(!IsNull(jni, o)) << "GetObjectField returned NULL";
+  return o;
+}
+
+jobject GetNullableObjectField(JNIEnv* jni, jobject object, jfieldID id) {
+  jobject o = jni->GetObjectField(object, id);
+  CHECK_EXCEPTION(jni) << "error during GetObjectField";
   return o;
 }