RecipientsPicker: Only list recipients with an email address

There is no point in showing those without one.

Also fix the setting of the To/Bcc default buttons to correspond
with the button labels and actions.
1 job for !11 with recipientspicker-only-with-email in 28 minutes and 48 seconds (queued for 3 seconds)
detached
Status Job ID Name Coverage
  Build
passed #66313
linux kf5-qt5 qt5.12

00:28:48

35.0%