Disallow the selection of variables in a negative branch

[Marie-Sparna]

Error 500 when activating the eye on "enableNegation" parameter. Not very useful to activate the eye in that case but we don't really understand the error when we run the query then :

[tfrancart]

Not very useful to activate

I would even say that selecting something which be definition does not exist is a logical non-sense :-)

but we don't really understand the error

The error message is given by Virtuoso, not by Sparnatural. Other triplestores would not return an error, and simply not bind the variable. We can't do anything about the error message.

What we can do however is to:

• if a "negation" is selected, then unselect all variables in that branch
• disable the selection of variable in a branch that is within a negation

I think this was working this way before the port to Typescript