Some algorithms can't process an empty alignment

There is a possibility to get an alignment with empty rows, that can cause crashes.

Scenario:

1. Open "data/samples/CLUSTALW/COI.aln".
2. Open "_common_data/fasta/empty.fa" as sequences.
3. Select "Add -> Sequence from current project..." in the context menu, add empty sequences.
4. Remove all non-empty rows from the alignment.
   Current state: the alignment contains two rows, that contain gaps only.

To get a crash try to get a distance between these rows.

A conceptual solution should be offered.