bail out early if there are no notes

... which usually is the case, so don't unnecessarily create iterators and dive
into an empty mdds tree.

Change-Id: Iba345cd231d5911adefabc0584290834678a68f1

sc/source/core/data/column4.cxx

@@ -620,6 +620,9 @@ void ScColumn::CreateAllNoteCaptions()
 
 void ScColumn::ForgetNoteCaptions( SCROW nRow1, SCROW nRow2, bool bPreserveData )
 {
+    if (maCellNotes.empty())
+        return;
+
     if (!ValidRow(nRow1) || !ValidRow(nRow2))
         return;