Fix nil in listener generator test (#157)
Fix nil in listener generator test (#157)

Marshalling `nil` values into `null` seems to work in OSS, but not in google3. Just remove nulls, our configs should never have this.

Signed-off-by: Teju Nareddy <86db88dbc1cba831353c118b8473a42f6361b545@google.com>
GoogleCloudPlatform/esp-v2,GoogleCloudPlatform/esp-v2,GoogleCloudPlatform/esp-v2,GoogleCloudPlatform/esp-v2,GoogleCloudPlatform/esp-v2