groupBy fix: when a group's dimension value is NULL, we need to adapt
the raw sql (stage: NULL -> stage IS NULL)
typeMapper fix: a graphql type should be made non-nullable if was
indicated so + does not have a default value. our check on not having a
default value was limited to having a null defaultValue instead of
having a null or undefined defaultValue. This is a breaking change, but
all the queries that were providing a null value for these args were not
functioning anyway, and luckily in the FE we declared all queries adding
a `!` already.
