I'm not a lawyer, nor do I play one on TV, but I would think that the EVSE is "in service" after it has been purchased, installed, received the final inspection, power is supplied to it, and it is ready to use. It would be temporarily out-of-service if it was broken or otherwise inoperative, until repaired and returned to service. It could be permenetly removed-from-service at it's end-of-life. This should be independent of whether an EV is in service. In theory, an EVSE might serve 0 or more EVs, and an EV might be serviced by 1 or more EVSEs. I think the thing which protects the IRS is that one is losing money by buying an EVSE which they are not planning to use, since the IRS only pays 50%.
The instructions for Line 15 are particularly interesting: it appears you can claim the credit twice if you move during the tax year but install EVSE at both locations. This would lead me to believe that the credit is tied to the EVSE and the EV is not required to claim it.
However the instructions state: "Original use of the property began with you." In many cases of folks here original use of the property will begin at a later date by the taxpayer claiming the credit. I think this is their way of insuring the you intend to use the EVSE and will use it, not merely resell it. I would think that, especially when transactions border the year boundry, tha the IRS might be willing to accept that you are placing the EVSE in-service in preparation for using it with the soon-to-be-acquired EV.