Backport NODE-5289 to v4.x.

Parent Ticket Description:

In some environments using bundlers, the require/import of saslprep can result in it returning a truthy value, and thus missing the nullish check before calling the function. This ticket is to ensure that if it is not nullish, it is a function before calling it.

AC:

• Update the check for saslprep to include a check that it is a function type.