common: fix NPE in AbstractUidPrincipal#equals

fixes commit 410762e

Fixes: #7325
Acked-by: Paul Millar
acked-by: Albert Rossi
Target: master
Require-book: no
Require-notes: no

modules/common/src/main/java/org/dcache/auth/AbstractUidPrincipal.java

@@ -101,6 +101,11 @@ public boolean equals(Object other) {
         if (this == other) {
             return true;
         }
+
+        if (other == null) {
+            return false;
+        }
+
         if (!(this.getClass().equals(other.getClass()))) {
             return false;
         }