fill missing dates with other values than zero

**Is your feature request related to a problem? Please describe.**
When executing resultset.series(), `fillMissingDates` is true by default, and the values of the missing dates are filled with 0.
In some scenarios, I have a data with 0 as values, and it makes it hard to identify which is actual date and which is missed date in the result.
An ability to set the missing date's value to null or to -1 it could be useful to some charts such as nivo (in nivo `value: null` is considered empty)

**Describe the solution you'd like**
1. Add another option `missingDateValue`
2. `fillMissingDate` will be either boolean or an object describing { enabled: boolean, value: number | null | etc.. }

Start by locating resultset.series() and the existing fillMissingDates option. Trace how missing dates are represented and check the surrounding API tests, if present; done means callers can distinguish missing dates from real zero values by configuring a missing-date value such as null or -1.

Written by the indexing model from the issue text.