fix getCameras not work

[Jerry-qiqi]

navigator.mediaDevice.getUserMedia maybe is undefined

[mebjas]

thanks for the change.

[rlueder]

I'm getting an error when running npm run-script build on 2.2.1 which seems related to this change, output below:

src/html5-qrcode.ts:735:39 - error TS2774: This condition will always return true since this function is always defined.
Did you mean to call it instead?

735         if (navigator.mediaDevices && navigator.mediaDevices.getUserMedia) {
                                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Found 1 error.

The check itself is valid as it's used again on line 398 but TypeScript doesn't seem to like the fact we're checking for getUserMedia and not actually using it, a simple fix is to just add // @ts-ignore to the line just above it:

// @ts-ignore
if (navigator.mediaDevices && navigator.mediaDevices.getUserMedia) {
  return Html5Qrcode.getCamerasFromMediaDevices();
}