Fix handling of empty XDG environment variables

Closes #21132

According to the XDG Base Directory specification
(https://specifications.freedesktop.org/basedir-spec/latest/#variables),
empty values for these environment variables should be treated the same
as if they are unset. Specifically, for the instances changed in this
commit,

> $XDG_DATA_HOME defines the base directory relative to which
> user-specific data files should be stored. If $XDG_DATA_HOME is either
> not set **or empty**, a default equal to $HOME/.local/share should be
> used.

and

> $XDG_CACHE_HOME defines the base directory relative to which
> user-specific non-essential data files should be stored. If
> $XDG_CACHE_HOME is either not set **or empty**, a default equal to
> $HOME/.cache should be used.

(emphasis mine)

In addition to the case mentioned in the linked issue, all other uses of
XDG environment variables were corrected.

0 files changed, 0 insertions, 0 deletions