fix: description_hash as an optional param to create_invoice.

fixes https://github.com/lnbits/lnbits/issues/74

lnbits/wallets/lntxbot.py

@@ -1,4 +1,5 @@
 from os import getenv
+from typing import Optional, Dict
 from requests import post
 
 from .base import InvoiceResponse, PaymentResponse, PaymentStatus, Wallet
@@ -13,12 +14,16 @@ class LntxbotWallet(Wallet):
         self.auth_admin = {"Authorization": f"Basic {getenv('LNTXBOT_ADMIN_KEY')}"}
         self.auth_invoice = {"Authorization": f"Basic {getenv('LNTXBOT_INVOICE_KEY')}"}
 
-    def create_invoice(self, amount: int, memo: str = "", description_hash: bytes = b"") -> InvoiceResponse:
-        r = post(
-            url=f"{self.endpoint}/addinvoice",
-            headers=self.auth_invoice,
-            json={"amt": str(amount), "memo": memo, "description_hash": description_hash.hex()},
-        )
+    def create_invoice(
+        self, amount: int, memo: Optional[str] = None, description_hash: Optional[bytes] = None
+    ) -> InvoiceResponse:
+        data: Dict = {"amt": str(amount)}
+        if description_hash:
+            data["description_hash"] = description_hash.hex()
+        else:
+            data["memo"] = memo or ""
+
+        r = post(url=f"{self.endpoint}/addinvoice", headers=self.auth_invoice, json=data,)
         ok, checking_id, payment_request, error_message = r.ok, None, None, None
 
         if r.ok: