Fix for loading interval of markerline style from SLD

[MoritzMoreira]

Added second .firstChild() to get to the tag holding the actual gap value.

Graphic stroke node of .sld file of a marker line:

            <se:GraphicStroke>
               <se:Graphic>
                 <se:Mark>
                   <se:WellKnownName>circle</se:WellKnownName>
                   <se:Fill>
                     <se:SvgParameter name="fill">#54b04a</se:SvgParameter>
                   </se:Fill>
                   <se:Stroke>
                     <se:SvgParameter name="stroke">#3d8035</se:SvgParameter>
                     <se:SvgParameter name="stroke-width">1</se:SvgParameter>
                   </se:Stroke>
                 </se:Mark>
                 <se:Size>14</se:Size>
               </se:Graphic>
               **<se:Gap>
                 <ogc:Literal>36</ogc:Literal>
               </se:Gap>**
             </se:GraphicStroke>

In the test file, the interval value was in the tag, so the sld specification probably changed.

[MoritzMoreira]

The interval value will not actually be checked when loading this test file, because the SLD loading only allows one option at a time. I have made an issue for multiple placement options (#61246). I did not remove the vendor option here in case it is needed somewhere else.

[github-actions]

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