XPath replace where tag is non-existent or empty

Hi

I am consuming the XML below into a database using an ETL tool.  The ETL tool allows me to specify the xpath to the specific fields to grab from xml and insert into the db table.  As you can see from the xml below, some of the tags are empty or do not exist.  I need an xpath expression that can substitute empty or non-existent tags with a NULL in the db table for that field e.g. on Entity_ID 2222 the Postal_Code is not present so I would want the Postal_Code field in the db table to be populated with a DB NULL.  Equally, on Entity_ID 1111, where the City tag is present but empty, I would want a NULL in the City db table field. Any help is greatly appreciated.  Thanks in advance.

    <?xml version="1.0"?>
    <NewDataSet>
    <Entity_Data msdata:rowOrder="0" xmlns:msdata="urn:schemas-microsoft-com:xml-msdata" diffgr:id="Entity Data1" diffgr:hasChanges="inserted" xmlns:diffgr="urn:schemas-microsoft-com:xml-diffgram-v1">
    <Entity_ID>1111</Entity_ID>
    <Name>EntityName</Name>
    <Street>EntityStreet</Street>
    <City/>
    <State>EntityState</State>
    <Postal_Code>12345</Postal_Code>
    </Entity_Data>
    <Entity_Data msdata:rowOrder="1" xmlns:msdata="urn:schemas-microsoft-com:xml-msdata" diffgr:id="Entity Data2" diffgr:hasChanges="inserted" xmlns:diffgr="urn:schemas-microsoft-com:xml-diffgram-v1">
    <Entity_ID>2222</Entity_ID>
    <Name>EntityName</Name>
    <Street>EntityStreet</Street>
    <City>EntityCity</City>
    <State>EntityState</State>
    <Phone>555-555-5555</Phone>
    </Entity_Data>
    <Entity_Data msdata:rowOrder="4" xmlns:msdata="urn:schemas-microsoft-com:xml-msdata" diffgr:id="Entity Data5" diffgr:hasChanges="inserted" xmlns:diffgr="urn:schemas-microsoft-com:xml-diffgram-v1">
    <Entity_ID>3333</Entity_ID>
    <Name>EntityName</Name>
    <Street>EntityStreet</Street>
    <City>EntityCity</City>
    <State>EntityState</State>
    <Postal_Code>22222</Postal_Code>
    </Entity_Data>
    </NewDataSet>