Support array-access safe navigation (x?[y])

The same way the expression a?.b shortcuts evaluation and returns null if a is null, this enhancement is to have the equivalent a?['b'] return null for the same reason.

Code example:

var a = {:};
a.b; // ok
a['b'];  // ok
a?.b; // ok
a?['b']; // 3.3 - parsing error in ';' 

with new JexlBuilder().safe(false).strict(true)

Thanks!