SimpleSelector::notEmpty() can never return true; deprecate it in favour of isBlank() #138

Problem

QueryPath\CSS\SimpleSelector::notEmpty() joins its checks with &&, so it only returns true when the element, ID, classes, combinator, attributes, pseudo-classes and pseudo-elements are all set, which a simple selector can't have (it has at most one ID, and the combinator is usually unset). It is never true in practice, and nothing in src/ or tests/ calls it.

isBlank() is the working inverse and is what the engine uses (PseudoClass::relativeSelector()).

Proposal

It's public, so for 4.x: mark notEmpty() @deprecated in favour of ! isBlank(), and either fix it to be ! $this->isBlank() (also counting the combinator if that's intended) or leave it as is. Remove it in 5.0.

Found in the /simplify pass on #119 / #125.