Fixes numba#5152

An implicit assumption was being made about the length of the iterable
when refining with extend. Specifically, this assumtion was that the
iterable has lenth >= 2. The code would however fail if the length is 1,
so this commit fixes that. Tests included.

This allows an untyped List to be extended with an empty iterable in the
interpreter. This is results in a NO-OP and is probably unlikely to
occur often in practice but is fixed for completeness. Tests included.

A `numba.typed.List` that is refined failed to extend on an empty
iterable. This fixes that to be a NO-OP and includes tests.

The 'term' singleton may not have been the appropriate choice here.

This simplifies the logic needed to implement `extend`.