Geoserver app-schema: types that only have client properties attributes are always skipped

As part of the work to skip attributes that are not mandatory and empty, attributes that have no sourceExpression and xlink:href in the ClientProperty are skipped.
As a result, types like swe:TextBlock are always skipped, because it only has ClientProperty attributes.
The fix should also exclude attributes that has ClientProperty values other than xlink:href.

<xs:element name="TextBlock">
<xs:complexType>
<xs:complexContent>
<xs:extension base="swe:AbstractEncodingType">
<xs:attribute name="tokenSeparator" type="swe:textSeparator" use="required"/>
<xs:attribute name="blockSeparator" type="swe:textSeparator" use="required"/>
<xs:attribute name="decimalSeparator" type="swe:decimalSeparator" use="required"/>
</xs:extension>
</xs:complexContent>
</xs:complexType>
</xs:element>

<xs:element name="TextBlock">
<xs:complexType>
<xs:complexContent>
<xs:extension base="swe:AbstractEncodingType">
<xs:attribute name="tokenSeparator" type="swe:textSeparator" use="required"/>
<xs:attribute name="blockSeparator" type="swe:textSeparator" use="required"/>
<xs:attribute name="decimalSeparator" type="swe:decimalSeparator" use="required"/>
</xs:extension>
</xs:complexContent>
</xs:complexType>
</xs:element>

One way to do this is to add a tag like <skipIfEmpty> tag for the AttributeMapping, with the current behaviour by default (skip if no value).