handling of an empty list when writing Web Map Context

[bartvde@…]

When e.g. the wms_stylelist metadata is set to an empty string in the MAP file,
it creates an invalid Web Map Context document.

"wms_stylelist"   ""

In the example below this causes an empty StyleList, while a StyleList should
always contain a Style.

    <Layer queryable="0" hidden="0">
      <Server service="WMS" version="1.1.0" title="Geostreets">
        <OnlineResource xlink:type="simple" xlink:href="http://blabla&amp;"/>
      </Server>
      <Name>layer_1</Name>
      <Title>Geostreets</Title>
      <SRS>EPSG:28992</SRS>
      <FormatList>
        <Format>image/jpeg</Format>
        <Format current="1">image/png</Format>
      </FormatList>
      <StyleList>
      </StyleList>
    </Layer>

See also:
http://www.maptools.org/bugzilla/show_bug.cgi?id=290

[assefa]

Test adding for empty string for wms_stylelist. There might be other places 
with similar problems. Please file specifc bug for these cases.
Marking it as Fixed.