fix: files, ctx and restore point at the stores on an empty index

[vshulcz]

Clean machine, nothing indexed:

before: deja files anything       → no sessions mention "anything"
        deja ctx anything         → no session matches "anything"
        deja restore some/file.go → no replaced spans recorded for "some/file.go"
after:  … + — no agent history was found on this machine; `deja sources` shows where deja looked

emptyIndexHint already backs last, blame and search (#832); these three read as "looked, not there" when there was nothing to look in. Answers on a store with history are unchanged. Closes #834.