Don't install Pex-style lockfile when no requirements used (#15706)
Don't install Pex-style lockfile when no requirements used (#15706)

Closes https://github.com/pantsbuild/pants/issues/15705.

[ci skip-rust]
[ci skip-build-wheels]
benjyw/pants,benjyw/pants,pantsbuild/pants,benjyw/pants,benjyw/pants,benjyw/pants,pantsbuild/pants,benjyw/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants