Gui: Allow None parameter again in drag and drop methods in Python ViewProviderPy class

[marioalexis84]

@donovaly separate pull request acording to https://forum.freecad.org/viewtopic.php?p=625495&sid=fbd133d17eb6673b7df62527a4054cfb#p625495
Pull request related: #7236

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[berndhahnebach]

for feature branch PR_7507. Pipeline 643111783 was triggered at a53a256. All CI branches and pipelines.

[donovaly]

Thanks! I will merge after I could merge #7236

[donovaly]

Wait, this PR is for master. In https://forum.freecad.org/viewtopic.php?p=625495&sid=61fd6b73f879d0636baa47506e4a0cf6#p625495 I requested a PR for the 0-20 branch.

To avoid confusions, can you please make ONE PR for master fixing the bug. If necessary please make a second PR for further improvements. First when the changes are merged into master, we can take care for the backporting.

[marioalexis84]

@donovaly pull request changed to FreeCAD-0-20.
The bug in master is fixed in #7236

[berndhahnebach]

for feature branch PR_7507. Pipeline 645562980 was triggered at 600a644. All CI branches and pipelines.