[FIX] web_editor: deleting copied table causing a false error traceback

Reproduction:
1. Open LibreOffice Calc (linux), create an empty xls file
2. In LibreOffice, input random things as a 4 (rows) by 2 (columns)
table
3. Create a new note in Odoo, copy the table to it
4. Hold Shift and click the place right before the first cell (out of
the table but still in the editable area)
5. Delete the selected table, a trace back popped up

Fix: add an extra condition to make sure parentFragmentTr is not null
before unpacking it

Note: as you probably noticed, the cleared table does not retain the
original dimensions, e.g. it turns to a 2-row, 4-column table. This is a
deeper issue of how we restore the table, and another task will be
created for this. This PR is a first-aid patch to solve the error
message as it confuses the client a lot

opw-3255144

closes odoo/odoo#119674

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