Refactor change in PR#7687

In pytest-dev#7687 a bugfix was added
after NOTSET was changed to a singleton enum, which ended up breaking
ID generation when it checked for isinstance(Enum).

That change, however, did not update the code path in the same ID
generation when used as an idfn function.

This adjusts the logic to handle both cases.

src/_pytest/python.py

@@ -1311,6 +1311,10 @@ def _idval(
     nodeid: Optional[str],
     config: Optional[Config],
 ) -> str:
+    if val is NOTSET:
+        # Fallback to default. Note that NOTSET is an enum.Enum.
+        return str(argname) + str(idx)
+
     if idfn:
         try:
             generated_id = idfn(val)
@@ -1334,15 +1338,13 @@ 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):
         # Name of a class, function, module, etc.
         name: str = getattr(val, "__name__")
         return name
+
     return str(argname) + str(idx)