Previously the "transpose" function would panic if any of the given lists
were null or if any of the strings inside the given lists were null.
Null values _are_ invalid in those positions because we can't project null
values into the keys of the resulting map, but we should return explicit
error messages in those cases rather than causing a cty panic.
Signed-off-by: Martin Atkins <mart@degeneration.co.uk>
97026e4048