Make it official that content_missing accepts missing keys.

Summary:
The PG Storage provides that feature, and the Git loader uses it.
This commit documents that feature, adds it to the Mem Storage,
and changes test_content_missing to test that behavior.

Reviewers: Reviewers, ardumont

Reviewed By: Reviewers, ardumont

Subscribers: swh-public-ci

Differential Revision: https://forge.softwareheritage.org/D680