fix: RBAC requests without headers fail to be logged (#3170)

There are certain failing RBAC requests which do not have headers, and
when we try to log them, the code throws an exception because of course
the map of requests is null, and we can't access its elements.

RHCLOUD-36553

backend/src/main/java/com/redhat/cloud/notifications/auth/rbac/RbacClientResponseFilter.java

@@ -58,6 +58,10 @@ public void filter(final ClientRequestContext requestContext, final ClientRespon
      * {@code {header1=value1, header2=[value2, value3]}}.
      */
     private String headersToString(final MultivaluedMap<String, String> headers) {
+        if (headers == null || headers.isEmpty()) {
+            return "";
+        }
+
         final StringBuilder sb = new StringBuilder();
         for (final Map.Entry<String, List<String>> entry : headers.entrySet()) {
             sb.append(entry.getKey());