fix(RTCUtils): Cannot read property 'find' of undefined

This error happens in getCurrentlyAvailableMediaDevices

JitsiMediaDevices.js

     setAudioOutputDevice(deviceId) {
         const availableDevices = RTC.getCurrentlyAvailableMediaDevices();
 
-        if (availableDevices && availableDevices.length > 0) {
+        if (availableDevices.length > 0) {
             // if we have devices info report device to stats
             // normally this will not happen on startup as this method is called
             // too early. This will happen only on user selection of new device

modules/RTC/RTCUtils.js

             logger.info(`Disable HPF: ${disableHPF}`);
         }
 
-        availableDevices = undefined;
+        availableDevices = [];
         window.clearInterval(availableDevicesPollTimer);
         availableDevicesPollTimer = undefined;