Fix build for linux python wheel (#22801)
### Description
Fixes command for building Linux python packages by preventing an empty
`-p` command-line option from being passed to a subsequent build script:
https://github.com/microsoft/onnxruntime/blob/1f3b675453e8412e5c084bfb95997967d0c2eec2/tools/ci_build/github/linux/run_python_dockerbuild.sh#L37
### Motivation and Context
A recent [PR
](https://github.com/microsoft/onnxruntime/pull/22773)introduced a new
optional command-line option (`-p`) to pass custom python exe paths. We
need to check if the option is empty before forwarding the option to a
separate build script.
