scripts: Compare to `None` using `is` operator

This is a trivial change that replaces `==` operator with `is` operator, following PEP 8 guideline:

> Comparisons to singletons like None should always be done with is or is not, never the equality operators.

https://legacy.python.org/dev/peps/pep-0008/#programming-recommendations

Change-Id: I4f9f6c921e4158365d4e41965bfcd43b7a3c07e0