Correctly check for missing entities in AbstractedRepoApi::getEntities

In 790fa2b9, check for missing entities was introduced to the client-side
getEntities wrapper. Unfortunately, it did not match the convention introduced
over half a year earlier in 75056367, and thus instead of just not returning
an entry for missing entities, the unserializer would throw an exception.

Change-Id: I007b504377fe6a5f28b86d5d2ffd301f6926422b
Merge "Correctly check for missing entities in AbstractedRepoApi::getEntities"