fix(java/restclient): avoid IndexOutOfBoundsException for empty multi… #3010

Job	Run time
Gradle tests (samples/client/petstore/java/jersey2-java8)	1m 24s
Gradle tests (samples/client/petstore/java/native)	1m 12s
Gradle tests (samples/client/petstore/java/okhttp-gson)	1m 30s
Gradle tests (samples/client/petstore/java/okhttp-gson-group-parameter)	1m 7s
Gradle tests (samples/client/petstore/java/webclient-swagger2)	1m 24s
Gradle tests (samples/openapi3/client/petstore/java/jersey2-java8-special-characters)	1m 21s
Gradle tests (samples/client/petstore/java/jersey2-java8-localdatetime)	1m 19s
Gradle tests (samples/openapi3/client/extensions/x-auth-id-alias/java/jersey2-java8)	1m 18s
Gradle tests (samples/openapi3/client/petstore/java/jersey2-java8)	1m 22s
Gradle tests (samples/client/petstore/java/native-jakarta)	1m 16s
	13m 13s

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