[Python][Packaging] macOS wheel platform tag is malformed when deployment target is unset #50531

Describe the bug, including details regarding any error messages, version, and platform.

On macOS, ci/scripts/python_wheel_macos_build.sh constructs _PYTHON_HOST_PLATFORM before applying the default value for MACOSX_DEPLOYMENT_TARGET. When the script is run without that variable exported, the resulting wheel platform tag is macosx--<arch> even though the build then defaults to macOS 12.0.

Expected: the default produces macosx-12.0-<arch>.

Actual: the tag has an empty version component.

The packaging CI currently exports the deployment target, so this affects manual or local invocations that rely on the script default.

Component(s)

Python, Packaging