install: Fix bugs around empty directories

The conversion between internal list representations led to empty
directories being left behind when processing generator expressions in
the destination. This ensures the intended behavior of creating a
destination directory but installing nothing into it when no <dirs> are
specified, regardless of the presence or absence of genex.

Further, an explicit empty string <dir> (via `""`, or an undefined
variable) led to the current source directory being expanded as a file
to install, leading to the generation of an infinitely recursive
install command. Disallow this behavior, forcing projects to explicitly
specify the current source directory if that is their intention.

Fixes: #27568