gh-93035: [Enum] Fix IntFlag crash when no single-bit members (GH-93076)

`EnumType` attempts to create a custom docstring for each enum/flag, but that was failing with pathological flags that had no members (only multi-bit aliases).

Lib/enum.py

@@ -538,7 +538,7 @@ class EnumType(type):
         #
         # create a default docstring if one has not been provided
         if enum_class.__doc__ is None:
-            if not member_names:
+            if not member_names or not list(enum_class):
                 enum_class.__doc__ = classdict['__doc__'] = _dedent("""\
                         Create a collection of name/value pairs.