unexpected exceptions when empty non-null sequence group passed to AlignmentView constructors

The jalview.viewmodel.AlignmentViewport.getAlignmentView(..) methods throw exceptions when the current alignment view's selection group is non-null but contains no sequences. This problem with the datamodel was noticed when debugging JAL-1031 and JAL-1032.

The datamodel's semantics need to be properly defined for the case when the currrent selection contains no sequences - there are plausible interpretations (e.g. the group could be the result of an intersection of two groups that only yields a column selection but no intersecting sequences, for instance?). These need to be enumerated and the specification updated to support these cases, if need be.