Fix Issue 12114 - buildNormalizedPath returns null instead of "."

[Issue details](https://issues.dlang.org/show_bug.cgi?id=12114)

in short:
`buildNormalizedPath(".")` and `buildNormalizedPath("foo", "..")` should both return `.` instead of null
Nitpick: please insert a space before `(`. Ditto for `if` statements below.