[FIX] web: keep context on form dialog save&new by default

ISSUE ADDRESSED
- Open Project > Tasks > Gantt view
- Click on the button "Create"  of any cell to open a form dialog
- Fill the form and click on SAVE&NEW button
- Fill again the form and click on SAVE&CLOSE button
- Both tasks are created, but only the first has been planned, because the second one has lost a part of the context (plan dates...)

BEFORE
Opening a form view dialog without passing a _createContext method (see commit c14b17c4 where it was introduced) was still passing an empty object additionalContext argument when save&new was hit.
This was leading to not take into account the full context (see BasicModel._getContext).

AFTER
If no _createContext method is passed to a form view dialog, then no empty object additionalContext is passed to evaluate the context, resulting in the full context being used.

cherry-pick of: https://github.com/odoo/odoo/pull/75795/commits/73d1321268224e25ba4ac9927c95a83e5cd3c400



opw-2655195

closes odoo/odoo#77413

Signed-off-by: Mathieu Duckerts-Antoine <Polymorphe57@users.noreply.github.com>