nullImportHook = async () => {
  return freeze({
    imports: [],
    exports: [],
    execute: () => {},
  })
}
The "null" import hook is for use during policy generation; it prevents
importing of exit modules and is the recommended way to handle exit modules
when using `captureFromMap()` (the reason, in a nutshell: different use-cases
are sharing the same internal `importHook` implementation in
`@endo/compartment-mapper`)

@type {ExitModuleImportHook}
@internal