messagesReducer [nfc]: Fix a sketchy null check.

This causes Flow to mark the `$FlowFixMe` on
`EventUpdateMessageAction.orig_subject?` as an "unused suppression
comment", so, remove it. In fact, I don't think a `$FlowFixMe`
really belongs here; there's no question that `orig_subject` is
sometimes missing, if only because private messages don't have
topics. The problem is in working out when it's there and when it's
not.

As I point out in the ongoing discussion [1] linked from that TODO,
there's a particular line [2] in the server code at current `master`
that I believe ensures that `update_message` events won't come to us
with an empty string for `orig_subject`.

[1] https://chat.zulip.org/#narrow/stream/206-zulip-terminal/topic/subject.20always.20present.20in.20event/near/1098954
[2] https://github.com/zulip/zulip/blob/08d716c74175a8b63bf8eb1080381c77f3f853c6/zerver/views/message_edit.py#L157

src/actionTypes.js

@@ -324,8 +324,22 @@ type EventUpdateMessageAction = {|
   message_id: number,
   // TODO is it really right that just one of the orig_* is optional?
   orig_content: string,
-  // $FlowFixMe clarify orig_subject vs. other orig_*, and missing vs. empty
+
+  // TODO: The doc for this field isn't yet correct; it turns out that
+  // the question of whether `orig_subject` is present or not is
+  // complicated; see discussion at
+  // https://chat.zulip.org/#narrow/stream/206-zulip-terminal/topic/subject.20always.20present.20in.20event/near/1098954.
+  //
+  // We can be pretty sure of a few things, though:
+  // - it will not be present if the message doesn't have a topic
+  //   (i.e., if it's a private message)
+  // - it's guaranteed to be present if the topic did indeed change
+  // - it will never be an empty string, because the server doesn't
+  //   accept the empty string for a message's topic; it requires
+  //   clients to specify something like `(no topic)` if no topic is
+  //   desired.
   orig_subject?: string,
+
   orig_rendered_content: string,
   prev_rendered_content_version: number,
   rendered_content: string,

src/message/messagesReducer.js

@@ -110,7 +110,7 @@ export default (state: MessagesState = initialState, action: Action): MessagesSt
             subject_links: action.subject_links || oldMessage.subject_links,
             edit_history: [
               action.orig_rendered_content
-                ? action.orig_subject
+                ? action.orig_subject !== undefined
                   ? {
                       prev_rendered_content: action.orig_rendered_content,
                       prev_subject: oldMessage.subject,