refactored the metadata comparison to avoid objects refrence comparisions

Author: Adagedo

auth/src/main/java/io/grpc/auth/ClientAuthInterceptor.java

@@ -77,7 +77,7 @@ protected void checkedStart(Listener<RespT> responseListener, Metadata headers)
           // JWT. However, today it does not cache JWT and so we won't bother tring to cache its
           // return value based on the URI.
           Map<String, List<String>> latestMetadata = getRequestMetadata(uri);
-          if (lastMetadata == null || lastMetadata != latestMetadata) {
+          if (lastMetadata.equals(null) || !Objects.equals(lastMetadata, latestMetadata)) {
             lastMetadata = latestMetadata;
             cached = toHeaders(lastMetadata);
           }