Disallow Addresses with `null` address part

This change ensures that Address objects are always created with a valid
`address` part. If an Address contains only `personal` part then it is
silently skipped from parsed list.

Unit tests have been adjusted to this new requirement.

Removed inserting Address with `null` address component introduced in
commit 68cecb16.

Fixes #3652.

See: https://github.com/k9mail/k-9/pull/3739#issuecomment-441242481