t6000: fix a careless test library add-on.
t6000: fix a careless test library add-on.

It tried to "restore" GIT_AUTHOR_EMAIL environment variable but
the variable started out as unset, so ended up setting it to an
empty string.  This is now caught as an error.

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git