[FIX] web_editor: don’t delete table cell when its left cell has content

Reproduction:
1. Create a 1 by 3 table, input something in the first cell
2. Go to the second cell, hit backspace
3. The second cell is deleted and the third cell is moved to the second
cell

Note: in 15, we don’t have the table selector to remove a row of the
table but that is another issue

Fix: when we try to delete the table cell and the offset is 0, nothing
should be done. Also added a test to make sure the cell is unchanged
when we try to delete in an empty cell which has a cell with content
at the left. Also we skip the rejoin block step if there's a table cell
element is invoivled because the cells are not deleted and rejoining
them is not needed

Task-3171904

closes odoo/odoo#114895

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>