Default initialize the tag to SPEND

TODO I would rather set it to None, so that we catch situations where it
is incorrectly initialized

Author: nielstron

pycardano/plutus.py

@@ -697,7 +697,7 @@ def __add__(self, other: ExecutionUnits) -> ExecutionUnits:
 
 @dataclass(repr=False)
 class Redeemer(ArrayCBORSerializable):
-    tag: RedeemerTag = field(default=None, init=False)
+    tag: RedeemerTag = field(default=RedeemerTag.SPEND, init=False)
 
     index: int = field(default=0, init=False)