Update RelatedField#field_from_native coding style

rest_framework/fields.py

@@ -351,13 +351,13 @@ class RelatedField(WritableField):
         if self.read_only:
             return
 
-        if field_name not in data and self.required:
-            raise ValidationError(self.error_messages['required'])
-        elif field_name not in data:
+        try:
+            value = data[field_name]
+        except KeyError:
+            if self.required:
+                raise ValidationError(self.error_messages['required'])
             return
 
-        value = data.get(field_name)
-
         if value in (None, '') and not self.null:
             raise ValidationError('Value may not be null')
         elif value in (None, '') and self.null: