fix(java/restclient): avoid IndexOutOfBoundsException for empty multi… #3011

Job	Run time
Gradle tests (samples/openapi3/client/extensions/x-auth-id-alias/java/jersey2-java8)	1m 31s
Gradle tests (samples/client/petstore/java/webclient-swagger2)	1m 24s
Gradle tests (samples/client/petstore/java/native-jakarta)	1m 15s
Gradle tests (samples/client/petstore/java/jersey2-java8-localdatetime)	1m 16s
Gradle tests (samples/openapi3/client/petstore/java/jersey2-java8-special-characters)	1m 19s
Gradle tests (samples/client/petstore/java/okhttp-gson-group-parameter)	1m 28s
Gradle tests (samples/openapi3/client/petstore/java/jersey2-java8)	1m 32s
Gradle tests (samples/client/petstore/java/jersey2-java8)	1m 36s
Gradle tests (samples/client/petstore/java/native)	1m 27s
Gradle tests (samples/client/petstore/java/okhttp-gson)	1m 30s
	14m 18s

Provide feedback