cam: options: Add empty() function to OptionValue class

Add a convenience helper to check if an option value is empty, based on
the value type. This is useful as a shortcut syntax to check if an
option has been set.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

src/cam/options.cpp

@@ -583,6 +583,13 @@ void OptionValue::addValue(const OptionValue &value)
  * \return The value type
  */
 
+/**
+ * \fn OptionValue::empty()
+ * \brief Check if the value is empty
+ * \return True if the value is empty (type set to ValueType::ValueNone), or
+ * false otherwise
+ */
+
 /**
  * \brief Cast the value to an int
  * \return The option value as an int, or 0 if the value type isn't