[html] prevent error when parsing an empty table

[midichef]

Parsing an empty table, like empty-table.html.txt,
using vd -f html empty-table.html.txt, causes this error:

File "/home/midichef/.local/lib/python3.10/site-packages/visidata/loaders/html.py", line 157, in iterload
it = list(list(x) for x in self.rows.pop(0))
IndexError: pop from empty list

This PR checks that the table isn't empty.