meson: udevdir might be an empty string

[jon-turney]

The following meson is of questionable validity:

https://github.com/systemd/casync/blob/5f4540fe4f42822a8a3beab982d040e69c294d9d/meson.build#L152

It's unclear that this is currently giving you what you want (an empty string), when udev.pc is not present. In future, it may be an error, especially if udev is found by a dependency provider other than pkg-config

See https://github.com/mesonbuild/meson/pull/4444#issuecomment-442443301 and following.

See also PRs #143, #157.