Volta won't pin tools if an empty volta configuration is present

[vallode]

For minimal reproduction:

1. Create a new npm package.json npm init --yes
2. Edit the package with an empty volta configuration "volta": {}
3. Attempt to pin anything with volta volta pin node
4. See error: error: No Node version found in this project.

Expected behaviour:

Volta should pin the default version of node, not throw an error.

[rwjblue]

Hmph. Thats annoying! Thanks for writing up a good issue with steps to reproduce.