Update 'qmk generate-api' to only publish pure DD keymaps

[zvecr]

Description

Avoid scenarios where keymap.json only contains build config and no layers, alongside a keymap.c which contains a typical keymap.

While maybe a better long term solution, updating list_keymaps was avoided to limit any snowballing of the change in behaviour.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).