Bail out of component chains if one returns null

Fixes issue 3903

Change-Id: I60820f7fa781c88377ccd0ef7339bd26badd4d81
Reviewed-on: http://gerrit.dmdirc.com/1066
Automatic-Compile: DMDirc Local Commits <dmdirc@googlemail.com>
Reviewed-by: Gregory Holmes <greg@dmdirc.com>

src/com/dmdirc/actions/ActionComponentChain.java

         Object res = argument;
         
         for (ActionComponent component : components) {
+            if (res == null) {
+                return null;
+            }
+            
             res = component.get(res);
         }