I cleared this place long before finding the quest to collect Gauldur's items (Forbidden Legend). When returning to Geirmund's, the marker shows where the amulet should be but there is nothing there. Perhaps whoever dropped it did not respawn - although most of the undead did, including the boss.

any ideas how I can complete this quest?

You should have been able to get the fragment the first time you went through I believe, I know that's what happened for me and whichever fragment gives you magicka.