Fix "No configuration for schema '' found" when route prefix is empty string

[hello-liang-shan]

Summary

Rebing\GraphQL\Exception\SchemaNotFound: No configuration for schema '' found is thrown if route prefix is empty string.

<?php

return [
    'route' => [
        'prefix' => '',
        ...
    ],
    ...
];

---

Type of change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Misc. change (internal, infrastructure, maintenance, etc.)

Checklist:

• Existing tests have been adapted and/or new tests have been added
• Add a CHANGELOG.md entry
• Update the README.md
• Code style has been fixed via composer fix-style

[mfn]

Hey there 👋🏼

Thank you, this LGTM!

[mfn]

Released as 8.2.0 🎉