SQLResult now gracefully handles adding a None value, ignoring the

value instead of raising an exception. This helps avoid issues and

simplifies code that otherwise had to check values before adding to the

result.
It also now handles Statement values, which are what most SQL

generation functions generate in Django 2.0 and higher. It will convert

them to SQL and ensure they have a trailing ;.
Both of these will be more necessary with upcoming support for

constraints in Django 2.2+.

Unit tests pass on all versions of Django, for all databases.
Tested both None and Statement handling in some in-progress changes.