invoice.get_amount_sat: handle None in more places

I believe lightning requests created before https://github.com/spesmilo/electrum/pull/7730
can have an amount of None - ones created after have amount 0 instead.
We could do a wallet db upgrade potentially.
Regardless, the type hint is `get_amount_sat(self) -> Union[int, str, None]`,
so None should be handled. (well, arguably "!" should be handled too...)

```
E | gui.qt.exception_window.Exception_Hook | exception caught by crash reporter
Traceback (most recent call last):
  File "...\electrum\electrum\gui\qt\request_list.py", line 101, in item_changed
    self.parent.show_receive_request(req)
  File "...\electrum\electrum\gui\qt\main_window.py", line 1279, in show_receive_request
    URI = req.get_bip21_URI(lightning=bip21_lightning)
  File "...\electrum\electrum\invoices.py", line 164, in get_bip21_URI
    amount = int(self.get_amount_sat())
TypeError: int() argument must be a string, a bytes-like object or a real number, not 'NoneType'
```

```
E | gui.qt.exception_window.Exception_Hook | exception caught by crash reporter
Traceback (most recent call last):
  File "...\electrum\electrum\gui\qt\request_list.py", line 101, in item_changed
    self.parent.show_receive_request(req)
  File "...\electrum\electrum\gui\qt\main_window.py", line 1281, in show_receive_request
    can_receive_lightning = self.wallet.lnworker and req.get_amount_sat() <= self.wallet.lnworker.num_sats_can_receive()
TypeError: '<=' not supported between instances of 'NoneType' and 'decimal.Decimal'
```

electrum/gui/qt/main_window.py

@@ -1278,7 +1278,7 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
         bip21_lightning = lnaddr if self.config.get('bip21_lightning', False) else None
         URI = req.get_bip21_URI(lightning=bip21_lightning)
         lightning_online = self.wallet.lnworker and self.wallet.lnworker.num_peers() > 0
-        can_receive_lightning = self.wallet.lnworker and req.get_amount_sat() <= self.wallet.lnworker.num_sats_can_receive()
+        can_receive_lightning = self.wallet.lnworker and (req.get_amount_sat() or 0) <= self.wallet.lnworker.num_sats_can_receive()
         if lnaddr is None:
             ln_help = _('This request does not have a Lightning invoice.')
             lnaddr = ''

electrum/invoices.py

@@ -161,7 +161,9 @@ class Invoice(StoredObject):
     def get_bip21_URI(self, lightning=None):
         from electrum.util import create_bip21_uri
         addr = self.get_address()
-        amount = int(self.get_amount_sat())
+        amount = self.get_amount_sat()
+        if amount is not None:
+            amount = int(amount)
         message = self.message
         extra = {}
         if self.time and self.exp:

electrum/lnworker.py

@@ -1098,7 +1098,7 @@ class LNWallet(LNWorker):
 
     def can_pay_invoice(self, invoice: Invoice) -> bool:
         assert invoice.is_lightning()
-        return invoice.get_amount_sat() <= self.num_sats_can_send()
+        return (invoice.get_amount_sat() or 0) <= self.num_sats_can_send()
 
     @log_exceptions
     async def pay_invoice(
@@ -2126,7 +2126,7 @@ class LNWallet(LNWorker):
 
     def can_receive_invoice(self, invoice: Invoice) -> bool:
         assert invoice.is_lightning()
-        return invoice.get_amount_sat() <= self.num_sats_can_receive()
+        return (invoice.get_amount_sat() or 0) <= self.num_sats_can_receive()
 
     async def close_channel(self, chan_id):
         chan = self._channels[chan_id]

electrum/submarine_swaps.py

@@ -419,7 +419,7 @@ class SwapManager(Logger):
             raise Exception("rswap check failed: locktime too close")
         # verify invoice preimage_hash
         lnaddr = self.lnworker._check_invoice(invoice)
-        invoice_amount = lnaddr.get_amount_sat()
+        invoice_amount = int(lnaddr.get_amount_sat())
         if lnaddr.paymenthash != preimage_hash:
             raise Exception("rswap check failed: inconsistent RHASH and invoice")
         # check that the lightning amount is what we requested

electrum/wallet.py

@@ -2167,7 +2167,7 @@ class Abstract_Wallet(AddressSynchronizer, ABC):
 
     def get_onchain_request_status(self, r: Invoice) -> Tuple[bool, Optional[int]]:
         address = r.get_address()
-        amount = r.get_amount_sat()
+        amount = int(r.get_amount_sat() or 0)
         received, sent = self.get_addr_io(address)
         l = []
         for txo, x in received.items():