[ExpressionLanguage] Fix dumping of null safe operator

[ivantsepp]

Q	A
Branch?	6.4
Bug fix?	yes
New feature?	no
Deprecations?	no
Issues	n/a
License	MIT

I noticed that when you dump an expression that uses null safe operator, the returned string doesn't maintain the null safe operator.

> (new ExpressionLanguage())->parse('user?.foo', ['user'])->getNodes()->dump() = "user.foo"

I would have expected user?.foo to be returned instead.

(I've also fixed some test data providers cases - getDumpData provider should only generate cases with 2 elements but some tests are including a third argument. So I went ahead and removed them)

[fabpot]

Thank you @ivantsepp.