Check for a behavior change on HttpHeaders.getRequestHeader(String)

This rule flags Java code that calls the javax.ws.rs.core.HttpHeaders.getRequestHeader(String) method because it returns a different value when the specified header does not exist.

Although the method signatures did not change, the method implementations return different values when the HTTP header specified in the parameter to this method does not exist. The jaxrs-2.0 and jaxrs-2.1 features return a null result when the header does not exist. The restfulws-3.0 feature returns an empty list when the header does not exist. These behavior changes do not cause compilation errors because the return type of the method is not changed. For further information, see Differences between Jakarta EE 9.1 and 8.0.