fix: wedo2 stop motor when power set to 0

[tpsnt]

When wedo2 motor power set to 0, the motor stops.

Resolves

When wedo2 motor power set to 0, the motor won't stop. And motorOff() won't stop the motors either.

Proposed Changes

This change adds a judge statement. When a startMotorPower() is invoked, it checks args.POWER. If args.POWER === 0, call a motor.turnOff first, then change the power attribute of the motor.

Reason for Changes

In WeDo2Motor.turnOff(), if this._power === 0, it returns immediately.
In Scratch3WeDo2Blocks.startMotorPower(), if args.POWER === 0, WeDo2Motor.turnOn() returns immediately.
So when you call Scratch3WeDo2Blocks.startMotorPower({POWER:0}), you won't turnOff() the motor, until you set the power to a number other and 0.

Test Coverage

Scratch3WeDo2Blocks.startMotorPower({POWER:0});
WeDo2Motor.turnOff();