Default AdjustedMatrix to use identity matrix.

The following discussion from !58 (merged) should be addressed:

• @jmfee started a discussion:

  This should still default to an identity matrix if no matrix was provided, and no statefile configured. Currently, this method returns immediately if no statefile is configured and may leave the matrix null.

mentioned in merge request !68 (merged)

added Needs Review label

marked the checklist item @jmfee started a discussion: as completed

closed with commit 645dde22

removed Needs Review label