An empty map shouldn't force a comma separator in join()

[nex3]

In Dart Sass, join(map-remove((c: d), c), 1 2 3) returns 1, 2, 3. This is incorrect—it should return 1 2 3, because an empty map should behave the same as an empty list and an empty list has no separator associated with it.

[nex3]

This is part of a larger issue where empty maps are always treated as having a comma separator. They should have an undecided separator, like the value () (to which they should be identical).