[html] fix failure from colspan with only td tags

[midichef]

When an HTML table has colspan, with only td tags, the visidata rows contain invalid data for the resulting empty cells, and the table can't be viewed.

More specifically, cells in a row created by the html loader should be tuples like (cellval, links):

visidata/visidata/loaders/html.py

Line 146 in d0e8a35

row[colnum] = (cellval, links)

But for empty cells, what iterload() adds to the row is a None object, not a tuple. That causes the following exception that makes the table unviewable:

Traceback (most recent call last):
  File "/home/j/.local/lib/python3.10/site-packages/visidata/threads.py", line 200, in _toplevelTryFunc
    t.status = func(*args, **kwargs)
  File "/home/j/.local/lib/python3.10/site-packages/visidata/sheets.py", line 235, in reload
    for r in self.iterload():
  File "/home/j/.local/lib/python3.10/site-packages/visidata/loaders/html.py", line 160, in iterload
    it = list(list(x) for x in self.rows.pop(0))
  File "/home/j/.local/lib/python3.10/site-packages/visidata/loaders/html.py", line 160, in <genexpr>
    it = list(list(x) for x in self.rows.pop(0))
TypeError: 'NoneType' object is not iterable

Here is an example file to reproduce the error: td_colspan.html.txt
It can be viewed with vd -f html td_colspan.html.txt

This PR fixes that issue and allows the table to be viewed.

The resulting table will still show errors for the empty cells, which I did not attempt to fix. They are the same errors already covered by #1308 , but caused by colspan instead of the rowspan that the open issue discusses.

[saulpw]

Thanks @midichef for catching this oversight!