Support 'None' Value in Enum Parser Variables

NetBrain's parser currently requires every enum variable to be defined with a fixed set of meaningful constants, with no built-in way to represent the absence of a value within that set. When a field may or may not contain a matching value, this forces users to build and maintain multiple separate paragraph parsers and merge their results into a single table. This enhancement would allow a 'None' or null option to be included as a valid member of an enum variable's defined constants, so the absence of a meaningful value can be represented directly within the enum. Supporting this would let customers apply constraints consistently on both sides of an enum variable and consolidate parsing logic into a single parser, reducing maintenance overhead and simplifying table construction.