or is there something special about a "." compared to a " "

No. The latter is special, the former is not.

When a single space is used as the first argument to split, it is treated specially, and is substituted for by a regex that is (approximately) equivalent to /\s+/.

This is a convenience token for a common case and (approximately) acts like a similar construct in another language.

There is no such special case for '.'.