Fetching a nil value with iterateIndexedValues(inColumn:) causes crash.

[MythicLionMan]

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