Currently, the signInActivity property isn't included for users who have never signed in. This means you also can't apply $filter to get such users.
Customers have requested the capability to filter users who have never signed in to help them manage inactive user accounts.
Other requests for the feature:
- Can I filter users who have no signInActivity? - Microsoft Tech Community
- Missing Data on signInActivity resource type · Issue #16823 · microsoftgraph/microsoft-graph-docs (github.com)