Enable strictNullChecks by default

[jtpio]

Fixes #71.

We should check whether we want to have strictNullChecks set to true by default in the cookiecutter for 3.0, as this might also require updating a couple of other places that depend on the cookiecutter and use the python -m jupyterlab.upgrade_extension script:

• apod tutorial: https://jupyterlab.readthedocs.io/en/latest/developer/extension_tutorial.html
• extension examples: https://github.com/jupyterlab/extension-examples

[jtpio]

cc @afshin what do you think about having this on by default for 3.0?

[fcollonval]

I think we could merge this one, no?

[jtpio]

Yes I think so too.