Null value handling in indexeddb

[vimwitch]

IndexedDB doesn't support null values in indexes. This problem is similar to #348.

To solve this problem nullish values should be avoided and instead empty strings or 0 values should be used as needed. Another approach is to add values indicating can more readily be queried.

This approach can be seen in the index-null-perf branch.

The main downside to this is new versions need to perform updates to existing databases converting null values to their corresponding values.