meson: udevdir might be an empty string

The following meson is of questionable validity:

Line 152 in 5f4540f

    
           udevdir = dependency('udev', required : false).get_pkgconfig_variable('udevdir')

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 mesonbuild/meson#4444 (comment) and following.