CwlPreconditionTesting allows us to unit test traps (i.e. fatalError,
assert, precondition). We currently don't have any way to test these in
the repo, so engineers simply don't test our assertions. This will
improve the overall state of our unit tests by encouraging testing of
more code paths.

Further reading on how CwlPreconditionTesting works: https://www.cocoawithlove.com/blog/2016/02/02/partial-functions-part-two-catching-precondition-failures.html

We currently don't validate that an ephemeral key is non-empty when a
user includes it in the `CustomerConfiguration`. This ends up giving
users a pretty obscure error in the API response, and is difficult to
debug today.

Including this validation will catch this issue early and provide users
a better debugging experience.

Co-authored-by: Yuki <[email protected]>

Co-authored-by: eurias-stripe <[email protected]>