Prevent rare NPE

app/src/main/java/eu/faircode/email/Core.java

@@ -1901,7 +1901,8 @@ class Core {
                     itrash.open(READ_WRITE);
                     try {
                         List<Message> trashed = new ArrayList<>();
-                        for (EntityMessage message : messages) {
+                        for (EntityMessage message : messages)
+                            if (!TextUtils.isEmpty(message.msgid)) {
                                 Message[] itrashed = itrash.search(new MessageIDTerm(message.msgid));
                                 if (itrashed != null && itrashed.length == 1)
                                     trashed.add(itrashed[0]);