Make not empty validator do a null check

Change-Id: Ic75627eee59724b171fe12ee92934e8b23fc79f7
Reviewed-on: http://gerrit.dmdirc.com/697
Automatic-Compile: Shane Mc Cormack <shane@dmdirc.com>
Reviewed-by: Shane Mc Cormack <shane@dmdirc.com>

src/com/dmdirc/config/prefs/validator/NotEmptyValidator.java

@@ -37,7 +37,7 @@ public class NotEmptyValidator implements Validator<String> {
     /** {@inheritDoc} */
     @Override
     public ValidationResponse validate(final String object) {
-        if (object.isEmpty()) {
+        if (object == null || object.isEmpty()) {
             return new ValidationResponse("Cannot be an empty string.");
         } else {
             return new ValidationResponse();