Improved empty empty alternatives detection.

[bd82]

The implementation of validateEmptyOrAlternative
https://github.com/SAP/chevrotain/blob/master/src/parse/grammar/checks.ts#L272

Does not take into account that an alternative may be empty indirectly.
For example:

$.OR([
     {ALT: () => { $.SUBRULE($.emptyRule)}}, /// emptyRule has no implementation. 
     {ALT: () => { $.CONSUME(Identifier)}}
     ]);

Another use case to take into account is:
What is an alternative is only sometimes empty?

$.OR([
     {ALT: () => { $.OPTION(() => { $.CONSUME(Integer)})}, 
     {ALT: () => { $.CONSUME(Identifier)}}
     ]);