[Java-OKHTTP-GSON] Do not set content-type if content type is null

[MarcMil]

The content type being null leads to an empty content type header. The resteasy server cannot process an empty content type:

java.lang.IllegalArgumentException: RESTEASY003340: Failure parsing MediaType string: 
	at org.jboss.resteasy.plugins.delegates.MediaTypeHeaderDelegate.internalParse(MediaTypeHeaderDelegate.java:96) ~[org.jboss.resteasy.core_4.7.1.Final.jar:?]
	at org.jboss.resteasy.plugins.delegates.MediaTypeHeaderDelegate.parse(MediaTypeHeaderDelegate.java:69) ~[org.jboss.resteasy.core_4.7.1.Final.jar:?]
	at org.jboss.resteasy.plugins.delegates.MediaTypeHeaderDelegate.fromString(MediaTypeHeaderDelegate.java:33) ~[org.jboss.resteasy.core_4.7.1.Final.jar:?]
	at org.jboss.resteasy.plugins.delegates.MediaTypeHeaderDelegate.fromString(MediaTypeHeaderDelegate.java:19) ~[org.jboss.resteasy.core_4.7.1.Final.jar:?]
	at javax.ws.rs.core.MediaType.valueOf(MediaType.java:172) ~[jakarta.ws.rs-api_2.1.6.jar:2.1.6]
	at org.jboss.resteasy.specimpl.ResteasyHttpHeaders.getMediaType(ResteasyHttpHeaders.java:151) ~[org.jboss.resteasy.core_4.7.1.Final.jar:?]
	at org.jboss.resteasy.specimpl.ResteasyHttpHeaders.testParsing(ResteasyHttpHeaders.java:69) ~[org.jboss.resteasy.core_4.7.1.Final.jar:?]
	at org.jboss.resteasy.plugins.server.servlet.ServletUtil.extractHttpHeaders(ServletUtil.java:83) ~[org.jboss.resteasy.core_4.7.1.Final.jar:?]
	at org.jboss.resteasy.plugins.server.servlet.ServletContainerDispatcher.service(ServletContainerDispatcher.java:226) ~[org.jboss.resteasy.core_4.7.1.Final.jar:?]
	at org.jboss.resteasy.plugins.server.servlet.HttpServletDispatcher.service(HttpServletDispatcher.java:60) ~[org.jboss.resteasy.core_4.7.1.Final.jar:?]
	at org.jboss.resteasy.plugins.server.servlet.HttpServletDispatcher.service(HttpServletDispatcher.java:55) ~[org.jboss.resteasy.core_4.7.1.Final.jar:?]

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[agilob]

👍

[wing328]

Tested locally with updated samples and the results are good.