tg.sh: make sure GIT_OBJECT_DIRECTORY is never set to ""

tg.sh: make sure GIT_OBJECT_DIRECTORY is never set to ""

Avoid exporting GIT_OBJECT_DIRECTORY unless it has a non-empty value.

GIT_OBJECT_DIRECTORY was always being marked for export, just in case
it had been fiddled with for alternate db purposes.

However, if it hasn't, and it's still unset (a very common occurrence),
a strictly pedantically interpreted reading of the POSIX standard
requires it to be placed in the environment EVEN IF IT'S UNSET!

This, of course, has the side-effect of setting it to the empty string!

Git checks the value using access and since `access("", X_OK)` always
fails (and Git is not smart enough to consider it unset if it's empty),
that pretty much guarantees immediate failure of all Git operations
that require a repository!

Fix this by removing the unintentional (aka BUG) exporting of an
unset or empty GIT_OBJECT_DIRECTORY and thereby eliminate a regression
introduced by the 0.19.7 release.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>