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
• Other (please describe)

Checklist

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