Security Group Name is empty for domain-joined Macs in SCCM:
This field is populated after completion of SCCM Active Directory Group Discovery:
Please make sure that Active Directory Group Discovery is enabled and configured properly.
In some cases it may fail for Macs that didn't contact domain for a long time:
Once this discovery succeeds for the Mac, SMS_AD_SECURITY_GROUP_DISCOVERY will be added in the Agent Name field and the System Group Name will be filled: