Issue #761 Padding reports for billing chart request

[Wedds]

This PR is related to issue #761

Previously, billing reports were not filling the whole requested period. For example, if users were active from 5th to 10th of a month, but the request is targeting the whole month, the response contained only the dates when users were active.

From now, if there are any spendings for the requested period, zero-cost spending docs are added for inactive dates.