apt-conf: uninitialised variables fix

In the bash script "99-override-default-apt-conf" 'set -u'
is being used to check for uninitialised variables.

This resulted in generic 'unbound variable' messages rather
than the intended error messages.

Tweaked the check for DIB_APT_CONF to work if the variable is unbound;
so that we get the intended error message rather than the `unbound
variable` message.

Change-Id: Ib8e287060f32312ab80dd7f44e2792ecca48edbb
Merge "apt-conf: uninitialised variables fix"