Fix NPE in createValue when content type is null

When build-with-parameters plugin calls createValue() during a GET
request (to render the parameter form), the request has no content
type. This causes req.getFileItem2() to pass null into
Pattern.matcher() inside commons-fileupload2's isMultipartRelated(),
triggering a NullPointerException.

The existing catch block only handles ServletException wrapping
FileUploadContentTypeException, so the NPE escapes.

Return null early when the content type is null, matching the
existing behavior for non-multipart requests.

Author: Axonis

src/main/java/io/jenkins/plugins/file_parameters/AbstractFileParameterDefinition.java

@@ -64,6 +64,9 @@ protected Object readResolve() {
     }
 
     @Override public ParameterValue createValue(StaplerRequest2 req) {
+        if (req.getContentType() == null) {
+            return null;
+        }
         try {
             FileItem src;
             try {