Allow nullable odor index

[bruno-f-cruz]

This would allow an experiment to be specified in such a way that it would still toggle the end valves (important to ensure that the sound is reproduced) but would not require a dummy value in the index to be encoded.

Aind.Behavior.VrForaging/src/DataSchemas/aind_behavior_vr_foraging/task_logic.py

Lines 76 to 78 in 1bc757e

	class OdorSpecification(BaseModel):
	index: int = Field(..., ge=0, le=3, description="Index of the odor to be used")
	concentration: float = Field(default=1, ge=0, le=1, description="Concentration of the odor")

[bruno-f-cruz]

The idea would be to simply set all odors to 0 concentration

[bruno-f-cruz]

This should be tackled at the same time as #263. I will defer this one until an experiment requires mixtures of odors.