Since the API doc doesn't specify what to do with 0 len writings into UART, we cannot assume that is safe to do it, so this commit prevents it by checking the length prior to the `uart_write()` operation.
Since the API doc doesn't specify what to do with 0 len writings into UART, we cannot assume that is safe to do it, so this commit prevents it by checking the length prior to the `uart_write()` operation.