fix!: require title when definitions are empty

[ericszentivanyi]

This PR introduces a new condition that requires a root schema to contain a title when its definitions are empty.

Previously, if this condition was not met, Typify would silently generate an empty output containing only boilerplate. This now results in an error, as a title or definitions are the bare minimum required to generate types.

We can assume that any schema meeting this condition is at least a partially valid JSON Schema for Typify’s purposes. The schemars crate does not provide any mechanism for validating schemas, so this can be used as a reasonable heuristic instead.

Fixes #516, fixes #313