Fix a warnings

[Taritsyn]

Fixed the following warnings:

warning CS8618: Non-nullable property 'SourceMapRoot' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.
warning CS1574: XML comment has cref attribute 'Nested' that could not be resolved
warning CS1574: XML comment has cref attribute 'SourceMap' that could not be resolved
warning CS1574: XML comment has cref attribute 'OutputFile' that could not be resolved
warning CS1574: XML comment has cref attribute 'SourceMapFile' that could not be resolved
warning CS1574: XML comment has cref attribute 'OutputFile' that could not be resolved
warning CS1574: XML comment has cref attribute 'OutputFile' that could not be resolved

[madskristensen]

Thanks