Undefined dates causing ValueErrors

[dylms]

Because start and today dates = None when undefined, creating or changing dates/scales of a project in the treeview occur a ValueError due to None - datetime.datetime objects comparisons in gantt processing.

Traceback (most recent call last):
  File "planning\gui\centralwidget.py", line 241, in <lambda>
  File "planning\gui\centralwidget.py", line 320, in update_planning_charts
  File "planning\model.py", line 1807, in generate_current_chart
  File "planning\model.py", line 795, in make_svg
  File "planning\gantt.py", line 2724, in make_svg_for_resources
  File "planning\gantt.py", line 2142, in _svg_calendar
TypeError: '>=' not supported between instances of 'NoneType' and 'datetime.date'

{C:\Users\...\proj\PyPlanning_base\planning\utils\qthelpers.py:161} CRITICAL - Unhandled exception
Traceback (most recent call last):
  File "C:\Users\...\proj\PyPlanning_base\planning\gui\centralwidget.py", line 237, in <lambda>
    lambda _ditem: self.update_planning_charts(self.planning)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\...\proj\PyPlanning_base\planning\gui\centralwidget.py", line 314, in update_planning_charts
    planning.generate_current_chart(index)
  File "C:\Users\...\proj\PyPlanning_base\planning\model.py", line 1764, in generate_current_chart
    chart.make_svg(project, one_line_for_tasks)
  File "C:\Users\...\proj\PyPlanning_base\planning\model.py", line 785, in make_svg
    project.make_svg_for_tasks(
  File "C:\Users\...\proj\PyPlanning_base\planning\gantt.py", line 2382, in make_svg_for_tasks
    dwg.add(self._svg_calendar(maxx, pheight, start_date, today, scale, offset=offset, t0mode=t0mode))
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\...\proj\PyPlanning_base\planning\gantt.py", line 2055, in _svg_calendar
    is_it_today = today >= jour and today < jour_dapres
                  ^^^^^^^^^^^^^
TypeError: '>=' not supported between instances of 'NoneType' and 'datetime.date'

[dylms]

Fix : enforce first task's start date when setting start and today date and adding new chart in tree

[dylms]

Fix for today: None check