fix: skip interpreters with empty output for WSL2 cross-compile

[plusls]

Summary

• Skip Python interpreters that exit with empty output when checking executable metadata
• Separate handling for pyenv "command not found" vs empty stderr cases with different warning messages

Problem

When cross-compiling from WSL2 to Windows (e.g. using cargo-xwin), PATH may include Windows Store Python stubs at:

/mnt/c/Users/<user>/AppData/Local/Microsoft/WindowsApps/python.exe

These stub executables exist but don't function as real Python interpreters - they just exit with an error code and produce no output (empty stderr). This caused maturin to fail with:

💥 maturin failed
  Caused by: Trying to get metadata from the python interpreter 'python.exe' failed

Solution

Gracefully skip interpreters with empty stderr output instead of failing the build. This allows cross-compilation workflows (like maturin build --target x86_64-pc-windows-msvc in WSL2) to proceed when using generate-import-lib feature.

Test Plan

Manually tested cross-compilation from WSL2 to Windows with cargo-xwin:

maturin build --release --sdist -o dist --target x86_64-pc-windows-msvc

Fixes #3135

[messense]

Thanks!