crossprobing pcb->schematic: handle correctly empty sheets.

(filtering selection candidates was not made for empty sheets)
Fixes #12427
https://gitlab.com/kicad/code/kicad/issues/12427

eeschema/cross-probing.cpp

@@ -553,6 +553,12 @@ bool sheetContainsOnlyWantedItems(
     SCH_REFERENCE_LIST references;
     aSheetPath.GetSymbols( references, false, true );
 
+    if( references.GetCount() == 0 )    // Empty sheet, obviously do not contain wanted items
+    {
+        aCache.emplace( aSheetPath, false );
+        return false;
+    }
+
     for( unsigned ii = 0; ii < references.GetCount(); ii++ )
     {
         SCH_REFERENCE& schRef = references[ii];