Fetching a nil value with iterateIndexedValues(inColumn:) causes crash. #527
Description
Unlike objects, the values in a YapDatabaseSecondaryIndexTransaction may be nil. The nullability specifier on enumerateIndexedValues:inColumn:matchingQuery:using: and its Swift counterpart are incorrect however. This means that calling the method from swift when a column value results in nil will cause a crash.
I've pushed a fix for this issue in PR #525