fix(mlir/**.py): fix comparison to None (#94019)

from PEP8
(https://peps.python.org/pep-0008/#programming-recommendations

):

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

Co-authored-by: Eisuke Kawashima <e-kwsm@users.noreply.github.com>