add value type invalid to vehicle_data proto

[patrickdemers6]

Description

Adds invalid to the vehicle_data proto. A data value is invalid when it is not available (e.g. MinutesToArrival will be invalid when no route is active).

Type of change

Please select all options that apply to this change:

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Bug fix (non-breaking change which fixes an issue)
• Documentation update

Checklist:

Confirm you have completed the following steps:

• My code follows the style of this project.
• I have performed a self-review of my code.
• I have made corresponding updates to the documentation.
• I have added/updated unit tests to cover my changes.
• I have added/updated integration tests to cover my changes.