core: Validate that xattr names aren't empty

core: Validate that xattr names aren't empty

In the ostree-ext codebase the test fixture was generating xattrs
without the trailing NUL byte.  This caused confusing errors
later.  Change the dirmeta validator to catch this.

The way GVariant represents bytestrings, the trailing NUL is there
on wire/disk so it can be there in memory too, but `g_variant_get_bytestring()`
will just return an empty `""` string if actually the value
has a missing NUL.

Signed-off-by: Colin Walters <walters@verbum.org>