Previously, mu generated a fake message ID for messages without a
Message-ID header. This fake message ID allows these messages to show in
an --include-related query. However, if a message contained a Message-ID
header with the value equal to the empty string, we did not generate a
fake message ID in the index, and consequently, these messages failed to
appear in an --include-related query. This change uses a fake message ID
when the Message-ID header is absent _or_ empty.
f0bba8e1fa