Allow Sim to have Zero Robots

[sauravbanna]

Description

Problem

Right now, we have a check in sim that prevents us from clearing out all the robots in sim. In Sandbox mode, this means that we can remove all but the last robot.

We also rely on there being at least 1 robot on the field to properly update the latest timestamp for each Team. However, we get this timestamp with the overall Vision packet anyway and so should handle the case where we have no robots.

Changes

Removed this check to allow for 0 robots. Along with this, added a wrapper around the yellow_robots / blue_robots fields in WorldState to differentiate between an empty list of robots vs an unset field / default packet

Also, refactored to use the Vision timestamp directly instead of the robot timestamp.

Why

This makes it easier to reset the field in Sandbox Mode. It also lets us run sim with robots on just one team (eg: for tests that do not involve enemies)

Trying to do anything with 0 robots (like GC commands or plays) may lead to unexpected behavior, but I think it's reasonable to trust that the user won't be surprised by this.

Testing Done

Tested along with Sandbox Mode changes (#3815), specifically the Clear Field button. Works as expected and the field is cleared.

Resolved Issues

Length Justification and Key Files to Review

Review Checklist

It is the reviewers responsibility to also make sure every item here has been covered

• Function & Class comments: All function definitions (usually in the .h file) should have a javadoc style comment at the start of them. For examples, see the functions defined in thunderbots/software/geom. Similarly, all classes should have an associated Javadoc comment explaining the purpose of the class.
• Remove all commented out code
• Remove extra print statements: for example, those just used for testing
• Resolve all TODO's: All TODO (or similar) statements should either be completed or associated with a github issue