[BUG] route with only a slash may not work

[spacether]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

A route with only a slash "/" may not work
Reported by Scott Knick in Slack

It was producing an import to a non-existent module, and the import was invalid too (like import ./../something.

openapi-json-schema-generator version

OpenAPI declaration file content or url

Generation Details

Steps to reproduce

Related issues/PRs

Created from OpenAPITools/openapi-generator#15312

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