[FIX] barcodes: skip undefined keys in barcode value
[FIX] barcodes: skip undefined keys in barcode value

This fixes an issue occurring with Firefox.
Some events was buffered with an undefined value and it wasn't
possible to build the barcode value from these events.
See 'handle_buffered_keys' function.

So now, we simply skip this kind of event.
We consider this as 'special keys'.

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