The PCI manager needs to check for instance UUIDs mapped to
None when cleaning up it's internal claims/allocations maps.
Also adds a check so the claims dict doesn't map to None in
the first place. There was already a check like this when
setting a value in the allocations dict.
Closes-Bug: #1238374
Change-Id: I7ac010f7d50ad85c145e536cfd48e07c8f4602d3