class Ransack::Context

this context does not change the conditions of an earlier one.
Condition captures this when it is built, so a later search reusing
set, including to `false` to turn the feature off for one search. A
Falls back to the global option unless this search's own option was

def null_sentinel
  @null_sentinel.nil? ? Ransack.options[:null_sentinel] : @null_sentinel
end