[8.x] Mail empty address handling

[Synchro]

This PR is a follow-up to this discussion.

Short version: I have an optional bcc address set in config that all emailed messages are sent to. It works fine if that value is set, but if it's not, then Laravel just crashes. This PR makes all mail addressing functions (to, cc, bcc, replyTo, from) handle empty addresses cleanly. In the existing Laravel code, this code will fail if mail.bcc is not set in config:

Mail::to($user)
    ->bcc(config('mail.bcc'))
    ->send(new Message($user));

Error:

Attempt to read property "email" on null

The two patches (to setAddress() and hasRecipient()) use empty() to check the $address parameter, so that all empty/falsy values are handled, including false, [], null and ''.

Alternatives to this patch are not immediately obvious, for example, this doesn't work:

->bcc(config('mail.bcc', []))

because the config will only fall through to the default if the config value is null (missing from .env altogether), not just empty (present, but without a value, as is common in .env.example files). This works, but it's ugly and annoying to have to do every time, and you should not need to know the internals of the mail class in order to do this safely:

->bcc(config('mail.bcc') ?: []))

I note that there are no tests for bad inputs, only good ones, so I have added tests for these bad cases, and they fail without this patch.

I also noticed that there are no tests at all for the from() and hasFrom() methods, so I added a full set of those in the same style.

[Synchro]

Thank you!