- even though we don't do anything yet, the code was calling `.length`
on an object, which is `undefined`
- this fixes that by checking the length of the keys
- also moves the code block down so we can early-return if mailgun is
not configured
refs https://github.com/TryGhost/Toolbox/issues/363
- this copies over tests from `email-analytics-provider-mailgun` that
are more relevant here
- there is now duplication in tests across the two packages but this
will be resolved soon
- this test checks that the mailgun client respects the changes in
settings, which is something that we used to ask
`email-analytics-provider-mailgun` to do when the mailgun client was
made in that package
- since then, we've pulled it out, so we should move the test to the
`mailgun-client` library
refs https://github.com/TryGhost/Toolbox/issues/363
- this commit pulls all code involving the Mailgun client SDK into one
new package called `mailgun-client`
- this means we should be able to replace `mailgun-js` (deprecated) with
`mailgun.js` (the new, official one) without editing code all over the
place
- this also lays some groundwork for better testing of smaller
components
