Improves error messages when an empty key is generated

[gwardwell]

Description

In cases where FROID schema schema generation results in empty @key fields (such as when only __typename is used as a key), FROID would return an unhelpful message:

Syntax Error: Expected Name, found "}".

This PR catches that error and instead throws an improved error to better explain what's going on:

Failed to parse key fields "" for type "SomeType" due to error: Syntax Error: Expected Name, found "}". 

Type of Change

• [ ] Bug Fix
• [ ] New Feature
• [ ] Breaking Change
• [ ] Refactor
• [ ] Documentation
• [x] Other (please describe)

Checklist

• [x] I have read the contributing guidelines
• [x] Existing issues have been referenced (where applicable)
• [x] I have verified this change is not present in other open pull requests
• [x] Functionality is documented
• [x] All code style checks pass
• [x] New code contribution is covered by automated tests
• [x] All new and existing tests pass