Provide option to include null values for SiteToSite Reporting Tasks

In some implementations of the SiteToSite Reporting Tasks there are variables captured which could have null values.  In these cases those values are stripped out from the final output by default.  Recommend adding a configurable option for users to opt to include null values to help ensure a consistent schema if needed (with default option being to remove values for backwards compatibility).

Affected Reporting Tasks:

SiteToSiteProvenanceReportingTask

SiteToSiteStatusReportingTask

SiteToSiteBulletinReportingTask

SiteToSiteMetricsReportingTask