regression: cannot create lightning invoice that never expires

[ecdsa]

Description

if I try to create a LN invoice without expiry, I get this:

  File "/opt/electrum/electrum/lnworker.py", line 140, in validate
    assert isinstance(self.expiry_delay, int) and self.expiry_delay > 0
AssertionError

this regression was introduced in 2b0cab6 not sure how we should handle it.