Fix Virtual layers display with empty subsetstring

Fixes #19721

Virtual layers incorrectly show no feature when the expression filter string is empty, instead of correctly showing all the features.

Subset string needs to be tested against Empty instead of Null in order to correctly populate the wheres list and prepare a well formed SQL query string.

(cherry picked from commit 99696b6)

src/providers/virtual/qgsvirtuallayerfeatureiterator.cpp

@@ -65,7 +65,7 @@ QgsVirtualLayerFeatureIterator::QgsVirtualLayerFeatureIterator( QgsVirtualLayerF
     QStringList wheres;
     QString offset;
     QString subset = mSource->mSubset;
-    if ( !subset.isNull() )
+    if ( !subset.isEmpty() )
     {
       wheres << subset;
     }