MDL-84213 reportbuilder: add course "do not force" field options.

MDL-84213 reportbuilder: add course "do not force" field options.

Where course entity select elements theme, language and calendar are
defined we should prepend with "Do not force" in order to match the
interface when editing the same fields.

The select filter has been updated to ensure it supports empty values
when switched to simplified version (a0ef4bb8) as well as improving
validation to ensure only present options can be used for filtering.