Qt: No amount entered isn't handled in "Receive Payjoin" dialog

[kristapsk]

Will end up with exception message in console and forever "waiting" state in UI.

Traceback (most recent call last):
  File "/home/user/joinmarket-clientserver/scripts/qtsupport.py", line 1051, in start_generate
    if not self.action_fn():
  File "/home/user/joinmarket-clientserver/./scripts/joinmarket-qt.py", line 1755, in startReceiver
    amount = btc.amount_to_sat(
  File "/home/user/joinmarket-clientserver/src/jmbitcoin/amount.py", line 27, in amount_to_sat
    raise ValueError("Invalid BTC amount string " + amount_str)
ValueError: Invalid BTC amount string