module: ensure `format` is never `null`
This PR ensures that the format
value for module resolution will always be either a string
or undefined
, never null
.
Further context for this change can be found on Slack: https://openjs-foundation.slack.com/archives/C019Y2T6STH/p1715696560302379. Essentially, returning a null
value for the format
property is not documented or expected.
(CC) @GeoffreyBooth (CC) @aduh95