fix: add additional check for navigator.keyboard

[nytamin]

I noticed this error pops up in our Sofie logs from time to time:

Client error, Sorensen.init, TypeError: TypeError: Cannot read properties of undefined (reading 'addEventListener'), TypeError: Cannot read properties of undefined (reading 'addEventListener')

The error originates from https://github.com/nrkno/sorensen/blob/main/src/index.ts#L661

I have no more info than this, but it seems that navigator.keyboard must have been set, but have the value undefined in some browser?

Anyway, this fix should fix that issue (fingers crossed).