python: fix empty parametrize() leading to "NotSetType.token" id

In ff8b7884e8 NOTSET was changed to a
singleton enum, which ended up unexpectedly triggering a code path in ID
generation which checks for `isinstance(Enum)`.

Add an explicit case for it, which is not too bad anyway.

src/_pytest/python.py

@@ -1293,6 +1293,9 @@ def _idval(
         return str(val)
     elif isinstance(val, REGEX_TYPE):
         return ascii_escaped(val.pattern)
+    elif val is NOTSET:
+        # Fallback to default. Note that NOTSET is an enum.Enum.
+        pass
     elif isinstance(val, enum.Enum):
         return str(val)
     elif isinstance(getattr(val, "__name__", None), str):