Throw ClassNotFoundException when appropriate

Currently, the ClassSource returned by ClassSource.fromMap will return null if the Class cannot be found (as that is the behavior of maps). However, other ClassSources throw a ClassNotFoundException if the class cannot be loaded. This commit changes the behavior of ClassSource.fromMap to throw a ClassNotFoundException if the class was not found in the map (or was mapped to null). This commit also changes the method to interpret a null map as an empty map.