A map adapter or maplibregl.Map; undefined to use Core's.
The resolved map, or null when none is available.
S13 — this used to return any truthy argument unchanged, so ensureMap("foo")
yielded "foo": the function ensured nothing, and the failure surfaced later at the
first method call, far from the cause. It now validates, so a non-map yields null
like an absent map. Both in-tree callers already handled null.
Resolves a usable map instance: the explicit one when it is a map, otherwise the one held by
Core, otherwisenull.