DOCSP-9826 doc updates for null values in attribute path (#21)

* DOCSP-9826 doc updates for null values in attribute path

Author: kanchana-mongodb

source/supported-unsupported/supported-partition-attributes.txt

@@ -124,3 +124,22 @@ for each data type:
    {+adl+} supports the `Package Syntax 
    <https://golang.org/pkg/regexp/syntax/>`__ for regular expressions 
    in the path to the filename.
+   
+Parsing Null Values from Filenames 
+----------------------------------
+
+If there is an empty string (``""``) in the place of an attribute in the path 
+to the file, {+dl+} automatically parses it as a null value for all the 
+attribute types except ``string``. For example, consider the following |s3| 
+{+data-lake-store+}:
+
+.. code-block:: none 
+   :emphasize-lines: 3
+
+   /records/january/1.json
+   /records/february/1.json
+   /records//1.json
+
+For the path ``/records/{month string}/*``, {+dl+} does not add any 
+computed fields for the ``month`` attribute to documents generated 
+from the third record in the above store.