TypeError when serializing a Pluck field valuated to None
Using `fields.Pluck` on an attribute valuated to `None` raises a `TypeError`, cf. the example below.
```python
from marshmallow import Schema, fields
from dataclasses import dataclass

@dataclass
class Nested:
    key: str

@dataclass
class Main:
    nested: Nested

class NestedSchema(Schema):
    key = fields.String()

class MainSchema(Schema):
    nested = fields.Pluck(NestedSchema, 'key')

m = Main(None)
dumped = MainSchema().dump(m)  # ERROR
# File ".../marshmallow/fields.py", line 521, in _serialize
#   return ret[self._field_data_key]
# TypeError: 'NoneType' object is not subscriptable
```
diff --git a/marshmallow/fields.py b/marshmallow/fields.py
index 238ec63..85fb57a 100644
--- a/marshmallow/fields.py
+++ b/marshmallow/fields.py
@@ -516,6 +516,8 @@ class Pluck(Nested):
 
     def _serialize(self, nested_obj, attr, obj, **kwargs):
         ret = super(Pluck, self)._serialize(nested_obj, attr, obj, **kwargs)
+        if ret is None:
+            return None
         if self.many:
             return utils.pluck(ret, key=self._field_data_key)
         return ret[self._field_data_key]
diff --git a/tests/test_schema.py b/tests/test_schema.py
index e950dd9..cadc402 100644
--- a/tests/test_schema.py
+++ b/tests/test_schema.py
@@ -2088,6 +2088,19 @@ class TestPluckSchema:
         for i, name in enumerate(data['collaborators']):
             assert name == blog.collaborators[i].name
 
+    def test_pluck_none(self, blog):
+        class FlatBlogSchema(Schema):
+            user = fields.Pluck(UserSchema, 'name')
+            collaborators = fields.Pluck(UserSchema, 'name', many=True)
+        col1 = User(name='Mick', age=123)
+        col2 = User(name='Keith', age=456)
+        blog = Blog(title='Unowned Blog', user=None, collaborators=[col1, col2])
+        s = FlatBlogSchema()
+        data = s.dump(blog)
+        assert data['user'] == blog.user
+        for i, name in enumerate(data['collaborators']):
+            assert name == blog.collaborators[i].name
+
     # Regression test for https://github.com/marshmallow-code/marshmallow/issues/800
     def test_pluck_with_data_key(self, blog):
         class UserSchema(Schema):
['tests/test_schema.py', 'marshmallow/fields.py']