[java] [spring] Fix multipart optional params error compilation using… #69

Job	Run time
Build Java Client JDK8 (samples/client/echo_api/java/okhttp-gson)	1m 0s
Build Java Client JDK8 (samples/client/echo_api/java/resteasy)	59s
Build Java Client JDK8 (samples/client/echo_api/java/apache-httpclient)	1m 15s
Build Java Client JDK8 (samples/client/echo_api/java/resttemplate)	1m 13s
Build Java Client JDK8 (samples/client/echo_api/java/feign-gson)	54s
	5m 21s

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