wallet: fix ergonomic issue in the multisig structure

Previously, it wasn't completely clear that MultisigRedeemScriptType.signatures needs to contain empty bytes for missing signatures. Now it can be shorter or even empty.

src/apps/wallet/sign_tx/scripts.py

@@ -156,13 +156,17 @@ def witness_p2wsh(
     signature_index: int,
     sighash: int,
 ):
-    signatures = multisig.signatures  # other signatures
-    if len(signatures[signature_index]) > 0:
+    # get other signatures, stretch with None to the number of the pubkeys
+    signatures = multisig.signatures + [None] * (
+        len(multisig.pubkeys) - len(multisig.signatures)
+    )
+    # fill in our signature
+    if signatures[signature_index]:
         raise ScriptsError("Invalid multisig parameters")
-    signatures[signature_index] = signature  # our signature
+    signatures[signature_index] = signature
 
     # filter empty
-    signatures = [s for s in multisig.signatures if len(s) > 0]
+    signatures = [s for s in multisig.signatures if s]
 
     # witness program + signatures + redeem script
     num_of_witness_items = 1 + len(signatures) + 1