Fix reading SLDs with self closing Filter

[KaiVolland]

This fixes reading of SLDs wit an empty filter. Compare the tests.

Please note @geographika.

[geographika]

The example SLD with an empty filter was generated from MapServer, so there are definitely cases where this could occur.
Thanks for the fix @KaiVolland !

[jansule]

This might be an error with MapServer, since according to the FilterEncoding specification, having empty filters is invalid.

<xsd:element name="Filter" type="ogc:FilterType"/>
<xsd:complexType name="FilterType">
  <xsd:choice>
    <xsd:element ref="ogc:spatialOps"/>
    <xsd:element ref="ogc:comparisonOps"/>
    <xsd:element ref="ogc:logicOps"/>
    <xsd:element ref="ogc:_Id" maxOccurs="unbounded"/>
  </xsd:choice>
</xsd:complexType>

See http://schemas.opengis.net/filter/1.1.0/filter.xsd

[KaiVolland]

IMHO the additional check will do no harm but we could make life easier for MapServer users. Maybe we could ad a lightweight SLD Validation to the parser.