Modex should ignore constraints on absent optional elements

Currently, a constraint return true iff:
- its element(s) is/are found;
- its function returns true.

This means that a constraint will return false if refering to an absent optional element. For example, the following rule will not match for a c:

```
a:b? c::TYPE:
+Constraint(left.1,"value")
```

This behavior should be changed as if we explicitly write that an element is optional then we probably want that constraints also allow its absence even if the constraint must be verified when the element is present.

When the change will be made, the documentation (md file) will have to be updated.