fix(ui): guard terminal writes while suspended

We forgot to guard `beep()` and `flush()`.

Ref: 36ec2b0ae1
Ref: dd76f9087f
Signed-off-by: Michael Vetter <jubalh@iodoru.org>
Fixes: https://github.com/profanity-im/profanity/issues/2162
(cherry picked from commit 5e4b5d313a9e055e039e2feb4efae58b6aaead78)

tests/unittests/ui/stub_ui.c

@@ -1475,3 +1475,13 @@ ui_is_suspended(void)
 {
     return FALSE;
 }
+
+void
+ui_beep(void)
+{
+}
+
+void
+ui_flash(void)
+{
+}