SQLFeatureStore: trying to remove attribute from feature results in NullPointerException

It is possible to remove a feature attribute by performing an update transaction that doesn't include a value for that attribute:

```xml
<?xml version="1.0" encoding="UTF-8"?>
<wfs:Transaction service="WFS" version="1.1.0"
    xmlns:app="http://www.deegree.org/app"
    xmlns:ogc="http://www.opengis.net/ogc"
    xmlns:wfs="http://www.opengis.net/wfs">
    <wfs:Update typeName="app:test">
        <wfs:Property>
            <wfs:Name>app:additionalInfo</wfs:Name>
        </wfs:Property>
        <ogc:Filter>
            <ogc:FeatureId fid="APP_TEST_1"/>
        </ogc:Filter>
    </wfs:Update>
</wfs:Transaction>
```
Within the SQLFeatureStore, this currently results in a NullPointerException. The patch proposed in this pull requests correctly implements this case.