It’s very rare for the caller address (as obtained by get_caller_address()) to be zero on Starknet. Let’s examine when can this happen.
The obvious case is when calling a view function. The call is not initiated by an account, so the caller is zero. But what about state changing functions?
In this case, the caller address is zero when it’s the StarknetOS executing the function. To the best of my knowledge, this happens only in the following scenarios: invoking __validate__ and __execute__ in an Account contract and handling an L1 -> L2 message in an #[l1_handler] function. Note that these functions are public, so anyone can invoke them, but when get_caller_address() is zero, you can be sure it’s a legit execution initiated by the protocol.
Thanks to @gaetbout, @martiray and @_tserg for their input when researching this piece.