DatabaseManager.get() - return `null` instead of `undefined` on empty result

[ondratra]

At the moment, DatabaseManager.get() function returns undefined when no record is found. This was in line with older TypeORM!s EntityManager that we were using until recently when we introduced a newer TypeORM version in #491.

Let's change the return value to null to align the returning value with EntityManager. The null value is a better representation of empty results anyway, plus easier to work with in JS.

After that, we need to update the code in Joystream QN to reflect this change.