Inconsistent null/empty str handling for multi-value/single-value dimensions in QueryableIndexStorageAdapter

[jon-wei]

In the Cursor returned by QueryableIndexStorageAdapter, the DimensionSelectors created from the Cursor have different null handling behavior for multi-value and single-value dimensions.

In the implementation of DimensionSelector.lookupName(id):

• For multi-value dimensions, Strings.nullToEmpty() is called on the value before it is returned or passed to the extractionFn.
• For single-value dimensions, there is no null->empty str conversion.

DimensionSelectors originating from an IncrementalIndex do not perform the null->empty str conversion either.

[xvrl]

@jon-wei @gianm can we put together a larger issue to aggregate all the null issues you've been seeing lately? It might make sense to have a more comprehensive view of all the issues to help figure out what needs to be done.

[jon-wei]

@xvrl I've opened an issue here: #2778

[jon-wei]

Fixed in #2690