Add null check

Author: wmontwe

mail/common/src/main/java/com/fsck/k9/mail/helper/Rfc822Token.java

@@ -205,6 +205,7 @@ public boolean equals(@Nullable Object o) {
     }
 
     public static String withULabelDomain(final String address) {
+        if (address == null) return null;
         int at = address.lastIndexOf('@');
         if (at < 0 || at == address.length()-1)
             return address;