Treat empty environment variables as unset

[philbooth]

Not sure what you'll think of this change, but it comes from a request from our ops folk. Essentially they were expecting an empty environment variable to fall back to the default value rather than overwrite it.

From an abundance of caution it's conditionally compiled behind a feature flag, as I presume it will be a breaking change for some people. Is there any chance of landing this (or something like it)?

[mehcode]

Hm. As I wrote a large chunk of this lib from reading through viper it definitely feels like an oversight on my part as viper ignores empty env vars.

And then we have bash:

❯ FOO= echo ${FOO:-20}
20

❯ echo ${FOO:-20}
20

Technically it's a behavioral breaking change but I'd probably file this under a bug. I'll merge this in a slight variation so that I can remove the config later without a breaking change. You'll want to update to 0.9.1 and do environment.ignore_empty(true) here: https://github.com/mozilla/fxa-email-service/blob/73d07bd7cc89181f759f7cbc99be2687f9b88331/src/settings/mod.rs#L386

If/when we decide to make this the default/only behavior we can softly deprecate that method. Removing a feature flag is a breaking change.

[philbooth]

Thanks @mehcode!