Fix a crash when rendering a raster outside of its extent

If QgsRasterLayerRenderer constructor exists prematurely because there
is nothing to render, its internal pointers end up null, but forceRasterRender()
was expecting that mPipe was always allocated.

The crash was introduced recently in #50382

src/core/raster/qgsrasterlayerrenderer.cpp

@@ -371,6 +371,9 @@ QgsFeedback *QgsRasterLayerRenderer::feedback() const
 
 bool QgsRasterLayerRenderer::forceRasterRender() const
 {
+  if ( !mRasterViewPort || !mPipe )
+    return false;  // this layer is not going to get rendered
+
   // preview of intermediate raster rendering results requires a temporary output image
   if ( renderContext()->testFlag( Qgis::RenderContextFlag::RenderPartialOutput ) )
     return true;