tests/test-xdg.t

templater: handle None returned from templatedir()

My recent 91aa9bba3dc9 (templater: make templatepaths() return a
single path, or None, 2020-07-21) didn't account for the fact that
`templatedir()` returns `None` in frozen binaries. That is ironic,
since the reason I'm working on this is to add support for built-in
mapfiles in frozen binaries. This patch updates the callers to handle
the `None` case. It's somewhat ugly, but I will have to revisit this
soon anyway, since my goal is to make all callers handle that case by
trying to read the map file using the resources API instead.

Differential Revision: https://phab.mercurial-scm.org/D8810

#if no-windows no-osx

  $ mkdir -p xdgconf/hg
  $ echo '[ui]' > xdgconf/hg/hgrc
  $ echo 'username = foobar' >> xdgconf/hg/hgrc
  $ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
  $ unset HGRCPATH
  $ hg config ui.username 2>/dev/null
  foobar

#endif