Allow null Files to be passed to TextFile again.

We do this for temporary configuration files which are never saved
(e.g. the addon settings). It's a bit of a kludge, but for now we
should maintain it.

Change-Id: I656b924221eded5bbd98dfa4c276ab6d43b682e0
Fixes-Issue: CLIENT-510
Reviewed-on: http://gerrit.dmdirc.com/3507
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>