Fix map and groupby filters ignoring default=None for missing attributes

[amanv0007]

Fixes #2165.

Problem

When using map(attribute="foo", default=None) on a list of objects that lack the foo attribute, the filter returns Undefined instead of None. This happens because make_attrgetter uses default is not None to decide whether to substitute Undefined values, making it impossible to distinguish "no default was given" from "the user explicitly passed default=None".

The same issue affects groupby which also passes default through to make_attrgetter.

Fix

Introduce a module-level _MISSING sentinel object and use it as the default value for the default parameter in make_attrgetter, prepare_map, and the groupby filter functions. The guard becomes default is not _MISSING instead of default is not None, allowing None to be a valid user-supplied default.

Testing

• All 911 existing tests pass without modification.
• • Manually verified that [{}] | map(attribute="foo", default=None) now correctly yields [None] instead of [Undefined].
• • Verified that default=0, default=False, and default="bar" still work correctly.
• • Verified that omitting default entirely still returns Undefined (unchanged behavior).