[Bug] [Java] Fix java compilation warnings in RFC3339JavaTimeModule a… #1167

Job	Run time
Build Java Client JDK11 (samples/client/echo_api/java/native)	1m 12s
Build Java Client JDK11 (samples/client/echo_api/java/feign-gson)	1m 12s
Build Java Client JDK11 (samples/client/echo_api/java/resttemplate)	1m 12s
Build Java Client JDK11 (samples/client/echo_api/java/apache-httpclient)	1m 13s
Build Java Client JDK11 (samples/client/echo_api/java/resteasy)	54s
	5m 43s

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