When a user specifies value overrides for list values out of order,
strvals.listItem panics. Change strvals.listItem to handle this case by
re-initializing nil values to a new map.

Closes #4503
Co-authored-by: Cameron Childress <cameron@cchildress.org>
Co-authored-by: Kevin Collette <hal.collette@gmail.com>
Co-authored-by: Connor McKelvey <connormckelvey@gmail.com>
Co-authored-by: Dan Winter <dan.j.winter@gmail.com>
Signed-off-by: Dan Winter <dan.j.winter@gmail.com>
Signed-off-by: Cameron Childress <cameron@cchildress.org>
Signed-off-by: Kevin Collette <hal.collette@gmail.com>
Signed-off-by: Connor McKelvey <connormckelvey@gmail.com>