Cleanup: use #isNotEmpty instead of #notEmpty

[astares]

We have many implementors of #isNotEmpty:

but we have also implementors of #notEmpty

The "isSomething" or "isNotSomething" is usually the better style within the method category "testing".

So we should

• convert all #notEmpty into #isNotEmpty
• deprecate the #notEmpty methods
• write a rule to check method selectors to point the user to the fact that it is better to prefix "notSomething" into "isNotSomething"