qgswfsgetcapabilities: Restore previous empty bounding box behavior

According to the WFS 1.1.0 specs "The <WGS84BoundingBox> element is
used to indicate the edges of an enclosing rectangle in decimal
degrees of latitude and longitude in WGS84" but the expected behavior
is not specified when the data are empty.

Up to QGIS 3.32, QGIS server returned:
```
<ows:WGS84BoundingBox dimensions="2">
<ows:LowerCorner>0 0</ows:LowerCorner>
<ows:UpperCorner>0 0</ows:UpperCorner>
```

However, as a side effect of a change in `QGSRectangle` (see:
qgis#54646), QGIS server now returns
since the 3.34 version:
```
<ows:WGS84BoundingBox dimensions="2">
<ows:LowerCorner>inf inf</ows:LowerCorner>
<ows:UpperCorner>-inf -inf</ows:UpperCorner>
```

This changes restores the QGIS 3.32 behavior.

src/server/services/wfs/qgswfsgetcapabilities.cpp

@@ -582,7 +582,7 @@ namespace QgsWfs
       //transform the layers native CRS into WGS84
       const QgsCoordinateReferenceSystem wgs84 = QgsCoordinateReferenceSystem::fromOgcWmsCrs( geoEpsgCrsAuthId() );
       const int wgs84precision = 6;
-      QgsRectangle wgs84BoundingRect;
+      QgsRectangle wgs84BoundingRect( 0, 0, 0, 0 );
       if ( !layerExtent.isNull() )
       {
         const QgsCoordinateTransform exGeoTransform( layer->crs(), wgs84, project );
@@ -592,7 +592,7 @@ namespace QgsWfs
         }
         catch ( const QgsCsException & )
         {
-          wgs84BoundingRect = QgsRectangle();
+          wgs84BoundingRect = QgsRectangle( 0, 0, 0, 0 );
         }
       }