Checking on null values in HTTP Header for protected JWS header

If a HTTP header is set but without a value, the current implementation causes a Nullpointer exception.

https://github.com/apache/cxf/blob/master/rt/rs/security/jose-parent/jose-jaxrs/src/main/java/org/apache/cxf/rs/security/jose/jaxrs/JoseJaxrsUtils.java

Line 79 needs to be changed from 

if (headerValues != null) {

to:

if (headerValues != null || headerValues.isEmpty() || headerValues.get(0) != null) {

In my case the header was set but without any values causing headerValues.get(0) to return null. The following code invokes .toString() on this null value causing the Nullpoiniter.