[vds] saving sheet with no rows makes following sheets unreadable

[midichef]

Steps to reproduce with sample data and a .vd
vd sample_data/sample.tsv no.txt sample_data/countries
Then save the sheets with save-all as unreadable.vds.
vd unreadable.vds will give an error.

File "/home/midichef/.local/lib/python3.10/site-packages/visidata/sheets.py", line 285, in loader
  for r in self.iterload():
File "/home/midichef/.local/lib/python3.10/site-packages/visidata/loaders/vds.py", line 73, in iterload
  raise Exception(d)
Exception: {'name': 'countries'}

The problem is that the .vds format can't specify a sheet with zero rows. The format has no marker for the end of a sheet.

Additional context
saul.pw/VisiData v3.1dev