most motherboards have safeties that will prevent a boot if the CPU_FAN header is empty. If you are water cooling with a custom loop you should put your radiator fan on the CPU header, if this is an AIO then follow the instructions of the AIO.
Normally, if there's no fan on the CPU fan header, it will ask you to enter BIOS on boot, and here you can set CPU Fan Speed to "Ignore".
After doing that, it will boot normally. That's the way I'm currently doing it, and I'm fairly certain this setting exist on most (quality) motherboards. I can't remember when I last had a board that didn't, but it's more than 10 years ago, so it's not a new or uncommon feature.