core: check null_or_empty for masked units instead of /dev/null

There's some inconsistency in the what is considered a masked unit:
some places (i.e. load-fragment.c) use `null_or_empty()` while others
check if the file path is symlinked to "/dev/null". Since the latter
doesn't account for things like non-absolute symlinks to "/dev/null",
this commit switches the check for "/dev/null" to use `null_or_empty_path()`