libcamera: controls: Default ControlList validator argument to nullptr

The ControlList constructor takes a validator pointer that can be null.
Set its default value to nullptr to simplify code in users of
ControlList.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Tested-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

include/libcamera/controls.h

@@ -126,7 +126,7 @@ private:
 	using ControlListMap = std::unordered_map<const ControlId *, ControlValue>;
 
 public:
-	ControlList(ControlValidator *validator);
+	ControlList(ControlValidator *validator = nullptr);
 
 	using iterator = ControlListMap::iterator;
 	using const_iterator = ControlListMap::const_iterator;