ArtifactoryImpl.restCall does not support requests without Content-Type

[yaroslav-orel-jfrog]

Describe the bug
Artifactory client does not support requests without Content-Type header. GET requests don't have Content-Type header and some endpoints in Artifactory do not expect this header to be provided (e.g. 'api/v1/service_trust/pairings/federated-repo/*'). As a result when executing such requests we get "message" : "Unsupported Media Type" because Client adds Content-Type: */* by default.

To Reproduce

Artifactory artifactory = ArtifactoryClientBuilder.create()
        .setUrl("ArtifactoryUrl")
        .setUsername("username")
        .setPassword("password")
        .build();
ArtifactoryRequest ar = new ArtifactoryRequestImpl()
                .method(ArtifactoryRequest.Method.GET)
                .apiUrl("api/v1/service_trust/pairings/federated-repo/*")
 artifactory.restCall(ar)

Expected behavior
Allow requests to be sent without Content-Type header

Additional context
In order not to break existing behavior, it would be safe to support requests with requestType == null. But Currently this line
ContentType contentType = Util.getContentType(artifactoryRequest.getRequestType());
throws NPE if requestType == null