Addons: differentiate sending invalid from empty version

[humitos]

We are currently not differentiating when the /_/addons/ endpoint receives any of the following:

• ?version= (empty)
• ?version=invalid-slug (invalid)
• ?url=https://docs.readthedocs.com/platform/invalid-slug/ (invalid)
• ?url=https://docs.readthedocs.com/platform/ (empty)

What would be the difference in these scenarios? cc @stsewd

Reference: #12372 (comment)