feat(rxFormOptionTable): Optional "empty" message for table

If the data array for an rxFormOptionTable, it will set a colspan
for a single row and fill it with `emptyMessage`. If `emptyMessage`
is not defined, it will default the previous behaviour of not
rendering the table at all.
@parlarjb Looking at Ernie's PR, I noticed these two lines don't really do much in the demo. Can they be removed?