Ethereum docs says:

However, occasionally validators might be offline when called to propose a block, meaning slots can sometimes go empty.

What happens when the randomly chosen validator is offline? Is the slot closed as empty, or is there a mechanism to choose a validator N number of times before the slot is considered empty?

Let's say there are 3 total validators(A, B and C), of which A is chosen to be the validator. But, A goes offline. Is the slot marked as empty, or is there an algorithm that chooses the next available validator(let's say B)?

Is there a heartbeat mechanism or a time-out mechanism when a validator is chosen? How does the network know the validator is alive? Let's say A was online when chosen by went offline right after it was chosen.