`PYTHON_LIBRARY` may be set to empty or NOTFOUND. (#61230)
Summary:
Not sure why (maybe from dependencies?) but it can certainly break package lookup upon re-entry of cmake.
So instead of checking whether they are defined, we should check whether there is any meaningful value inside.
Fixes https://github.com/pytorch/pytorch/issues/59887
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61230
Reviewed By: H-Huang
Differential Revision: D29668766
Pulled By: malfet
fbshipit-source-id: 79a59578740c4434327aff4f9a22eba9c4bf48d1
