In rC07722d89bac1: kdf,fips: Modify HKDF test for FIPS mode., I fixed HKDF tests for shorter salts for HKDF in FIPS mode.
However, no salt is actually a valid practice for use of HKDF.
For concrete reference, I found this one: https://pages.nist.gov/ACVP/draft-hammett-acvp-kas-kdf-hkdf.html
So, I think that rejecting is not good.