ComposeBox [nfc]: Use optional chaining instead of if block.

Prompted by discussion on zulip#4101 [1].

Flow started supporting method calls in optional chains in
v0.112.0 [2], but ESLint isn't yet on board [3]; it gives a false
positive for `no-unused-expressions`.

A comment on the ESLint issue [4] suggests we could be more
systematic by using an ESLint plugin from Babel, instead of one-off
suppressions of `no-unused-expressions` like this one. That plugin
moved from (on NPM) `babel-eslint-plugin` to `@babel/eslint-plugin`.
We can't use the new one until we're on ESLint 7 (see zulip#4254), but we
could probably use the old one.

[1] zulip#4101 (comment)
[2] facebook/flow#4303
[3] eslint/eslint#11045
[4] eslint/eslint#11045 (comment)

src/compose/ComposeBox.js

@@ -250,9 +250,9 @@ class ComposeBox extends PureComponent<Props, State> {
     this.setMessageInputValue(completedText);
 
     if (lastWordPrefix === '@') {
-      if (this.mentionWarnings.current) {
-        this.mentionWarnings.current.getWrappedInstance().handleMentionSubscribedCheck(completion);
-      }
+      // https://github.com/eslint/eslint/issues/11045
+      // eslint-disable-next-line no-unused-expressions
+      this.mentionWarnings.current?.getWrappedInstance().handleMentionSubscribedCheck(completion);
     }
   };