Fix spawning without standard handles on Windows

[cg-zhou]

Windows GUI applications may have no standard handles. In that case,
UpdateProcThreadAttribute receives an empty handle list and fails with
ERROR_BAD_LENGTH.

Skip setting PROC_THREAD_ATTRIBUTE_HANDLE_LIST when there are no handles to
inherit, and add a Windows regression test covering spawn_orphan without
standard handles.

Tested with:

• moon test src/process --target native — 42 passed
• The regression test reproduces ERROR_BAD_LENGTH before the fix and passes
  after it