My strategy is to just wait a bit and not respond immediately, to see if they get the message that their initial greeting was useless, and then post what they actually have to say.
In some cases, the sender deleted the hello, and then attempted another hello later.
In some cases, the sender would wait a few hours and say hello again, or some just email me instead (they must have assumed I am not available).
In one exceptional case, I waited 2 days before saying "hello" back, and they then immediately continued the conversation as though no abnormal communication gap had occurred.