Use nullptr in module visualization

[SunBlack]

Changes are done by run-clang-tidy -header-filter='.' -checks='-,modernize-use-nullptr' -fix

[taketwo]

I'm not sure if we should modify the "cxx" files from vtk/ directory. These were directly copied from VTK source tree. @SergioRAgostinho what do you think?

[SergioRAgostinho]

Do you have any idea on what's the story behind these files? Here's what I found

The history:
https://github.com/PointCloudLibrary/pcl/commits/84515237c3d4c692ccde559338b9e040828b401b/visualization/src/vtk/vtkVertexBufferObject.cxx

Some relevant commits:
Addition of the vtk source tree files - 2e5624d#diff-8e3e87eb9884b3bc2dc078e300a79c1d
The initial commit of this 2d canvas object - ff81eba#diff-5fe104abeeaaf3fa0b0c27ef1e5e5c64

Answering your question, I think we should approve the changes to these special files and simultaneously open an issue into investigating if these files can actually be stripped out of our source tree. I suspect that ultimately, we're gonna find that someone made some custom changes which won't allow us to easily remove them. If changes were indeed made and there are evidences suggesting that, then there's no reason to worry about further "tainting" the code with more changes.

[taketwo]

No, these files date back too far, I have no knowledge of them. But I agree with what you wrote.