invoices: get_outputs to use .outputs field if available

It is wasteful to create new PartialTxOutput objects if we already have an outputs field.
Btw apparently lightning invoices too have an outputs field.

electrum/invoices.py

@@ -1,5 +1,5 @@
 import time
-from typing import TYPE_CHECKING, List, Optional, Union, Dict, Any
+from typing import TYPE_CHECKING, List, Optional, Union, Dict, Any, Sequence
 from decimal import Decimal
 
 import attr
@@ -126,16 +126,13 @@ class Invoice(StoredObject):
             address = self._lnaddr.get_fallback_address() or None
         return address
 
-    def get_outputs(self):
-        if self.is_lightning():
+    def get_outputs(self) -> Sequence[PartialTxOutput]:
+        outputs = self.outputs or []
+        if not outputs:
             address = self.get_address()
             amount = self.get_amount_sat()
             if address and amount is not None:
                 outputs = [PartialTxOutput.from_address_and_value(address, int(amount))]
-            else:
-                outputs = []
-        else:
-            outputs = self.outputs
         return outputs
 
     def can_be_paid_onchain(self) -> bool: