Fixed missing mapping of required attributes if property is nillable

[lgoltz]

Using the Gml Tools SqlFeatureStoreConfigCreator the mapping of required attributes is missing if a property is nilled.

deegree-tools-gml.jar SqlFeatureStoreConfigCreator -idtype=uuid -useRefDataProps=true -format=all -deegree.gml.parse.recognize-deprecated-types=false -dbschema=alkis -referenceData=building.gml -schemaUrl=https://inspire.ec.europa.eu/schemas/bu-core2d/4.0/BuildingsCore2D.xsd

Example from https://inspire.ec.europa.eu/schemas/bu-core2d/4.0/BuildingsCore2D.xsd:

<complexType name="MeasureType">
  <simpleContent>
    <extension base="double">
      <attribute name="uom" type="gml:UomIdentifier" use="required"/>
    </extension>
  </simpleContent>
</complexType>

uom is a required attribute but missing in the resulting mapping:;

<Complex path="bu-base:horizontalGeometryEstimatedAccuracy">
  <Primitive path="@xsi:nil" mapping="bu_core2d_geometry2d_bu_base_buildinggeometry2d_bu_base_hori_24"/>
  <Primitive path="@nilReason" mapping="bu_core2d_geometry2d_bu_base_buildinggeometry2d_bu_base_hori_25"/>
</Complex>

The referenceData contains the property:

 <bu-base:horizontalGeometryEstimatedAccuracy uom="m" nilReason="other:unpopulated" xsi:nil="true"></bu-base:horizontalGeometryEstimatedAccuracy>

This PR fixes the Gml Tool by writing the mapping of required attributes even if the property is nilled:

<Complex path="bu-base:horizontalGeometryEstimatedAccuracy">
  <Primitive path="@xsi:nil" mapping="bu_core2d_geometry2d_bu_base_buildinggeometry2d_bu_base_hori_44"/>
  <Primitive path="@nilReason" mapping="bu_core2d_geometry2d_bu_base_buildinggeometry2d_bu_base_hori_45"/>
  <Primitive path="@uom" mapping="bu_core2d_geometry2d_bu_base_buildinggeometry2d_bu_base_hori_46"/>
</Complex>