Add label backup "unlabeled microphone" in case there is no label

Fixes #705

This backup is not i18ned, but (a) it's very rare it will be used and
(b) microphone names are not i18ned either.

src/ui/studio/audio-setup/index.js

@@ -161,7 +161,7 @@ const MicrophonePreview = ({ reselectSource, enterStudio }) => {
         >
           {
             devices.map((d, i) => (
-              <option key={i} value={d.deviceId}>{ d.label }</option>
+              <option key={i} value={d.deviceId}>{ d.label || "unlabeled microphone" }</option>
             ))
           }
         </select>