Only use immutable placeholder variables in RAVEN functions

There are numerous instances where we pass empty lists to the ligo-raven package, which is not the best design. In the future these should be switched to Nones.

changed milestone to %GWCelery v2.4.1 Release

added External Triggers label

assigned to @brandon.piotrzkowski

added to epic &12

changed the description

Noting that this requires the next release of ligo-raven , version 4.0.

Pulling from the next release since this requires a reviewed ligo-raven 4.0, which will not be ready in time.

A preliminary version is now available, so we can start working on getting this to work prior to the review finishing.

changed milestone to %GWCelery 2.6.0 Release

changed milestone to %GWCelery 2.6.1 release