Cosmetic refactoring

src/main/groovy/io/kamax/mxisd/invitation/InvitationManager.java

@@ -235,12 +235,14 @@ public class InvitationManager {
     }
 
     public void lookupMappingsForInvites() {
+        if (!invitations.isEmpty()) {
             log.info("Checking for existing mapping for pending invites");
             for (IThreePidInviteReply reply : invitations.values()) {
                 log.info("Processing invite {}", getIdForLog(reply));
                 ForkJoinPool.commonPool().submit(new MappingChecker(reply));
             }
         }
+    }
 
     public void publishMappingIfInvited(ThreePidMapping threePid) {
         log.info("Looking up possible pending invites for {}:{}", threePid.getMedium(), threePid.getValue());