feature #6594 Allowed to return null for query_builder (JonEastman)

This PR was squashed before being merged into the 2.8 branch (closes #6594).

Discussion
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 Allowed to return null for query_builder

| Q             | A
| ------------- | ---
| Doc fix?      |  yes
| New docs?     | no
| Applies to    | 2.8
| Fixed tickets | #5934

Commits
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f84a1eb  Allowed to return null for query_builder

Author: wouterj

reference/forms/types/entity.rst

@@ -202,9 +202,10 @@ query_builder
 Allows you to create a custom query for your choices. See
 :ref:`ref-form-entity-query-builder` for an example.
 
-The value of this option can either be a ``QueryBuilder`` object or a Closure.
+The value of this option can either be a ``QueryBuilder`` object, a Closure or ``null``.
 When using a Closure, you will be passed the ``EntityRepository`` of the entity
-as the only argument and should return a ``QueryBuilder``.
+as the only argument and should return a ``QueryBuilder``.  
+If you'd like to display a list of empty entries, you can return ``null`` in the closure.
 
 Overridden Options
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