Spoofing a serial number on a MacBook Pro
AFAIK, there is no way to clear a serial intentionally or by accident. The only way to get a blank system serial number is when the logic board is replaced. Replacement MLB only comes with a part serial number but it does not have a system serial number. I assume that you took your laptop to have it repaired?

When a new logic board is replaced by Apple or an Apple Certified Service Provider, the logic board needs to be serialized after the repair. Normally an ACMT doing the repair will run basic MRI test after the repair which will then be identified as a blank board thus allowing the ACMT to add a serial number to the board prior to running the test.

You can go back to the repair shop and they will most likely correct the sloppy error of forgetting to add the serial number. If that option is not available then you can try going to the closest Apple store but the only way they will help you is if Apple or an Apple store branch did the repair or an ACSP/ACMT did the repair.