Dcoument EmptyFormFieldDependency

Resolves #235

Author: TimWolla

docs/php/api/form_builder/dependencies.md

@@ -50,9 +50,10 @@ When the form is built, all dependencies that only know the id of the relevant f
 
 ## Default Dependencies
 
-WoltLab Suite Core delivers the following two default dependency classes by default:
+WoltLab Suite Core delivers the following dependency classes by default:
 
-- `NonEmptyFormFieldDependency` can be used to ensure that a node is only shown if the value of the referenced form field is not empty (being empty is determined using PHP’s `empty` function).
+- `NonEmptyFormFieldDependency` can be used to ensure that a node is only shown if the value of the referenced form field is not empty (being empty is determined using PHP’s `empty()` language construct).
+- `EmptyFormFieldDependency` is the inverse of `NonEmptyFormFieldDependency`, checking for `!empty()`.
 - `ValueFormFieldDependency` can be used to ensure that a node is only shown if the value of the referenced form field is from a specified list of of values (see methods `values($values)` and `getValues()`).
   Additionally, via `negate($negate = true)` and `isNegated()`, the logic can also be inverted by requiring the value of the referenced form field not to be from a specified list of values.