Removed dead code from Operator API
Refactored JSON deserialization for RequestStopTransaction
Removed magic string for date format
Added mock repo to test utils
Refactored JSON deserialization for ChangeConfiguration
Fixed potential null issues in Core API

- Made all parameters non-null by default (using an annotation on the package).
- Added @Nullable to all currently nullable parameters and return types. Some of these shouldn't be nullable to begin with but this will be fixed in a future commit.
- Added null checks where they were missing. Some older code didn't contain null checks but they were intended and reasonably safe to add.
Replaced nullable parameters with non-null empty values

Instead of nullable collections, empty collections are preferred. Instead of nullable strings, empty strings are preferred.