Yes, you can boot a system without an initrd image.
initrd image is either a gzipped ramdisc image, or (more commonly nowadays) a gzipped .cpio image.
In the latter case, the .cpio is expanded into a filesystem called initramfs.
If the .cpio image isn't present, the kernel uses a built-in image instead, which contains just a few special files (such as /dev/console, /dev/null and a few directories), but no binaries.
The kernel then uses some built-in logic and command-line options to try to find and mount is "real" root filesystem, which is mounted "over" the initramfs and therefore hides it.
This "legacy" boot system is mostly not used in modern distributions.
