fdo#51266: Properly set pivot item 'empty' on empty cells.

Or else it would re-use the last non-empty cell value which is wrong.

Change-Id: I3feab09a54e93150a7b6a9e9b4b540327f2ab5c5

sc/inc/dpitemdata.hxx

@@ -89,6 +89,7 @@ public:
     ~ScDPItemData();
 
     Type GetType() const;
+    void SetEmpty();
     void SetString(const rtl::OUString& rS);
     void SetString(const rtl::OUString* pS);
     void SetValue(double fVal);

sc/source/core/data/dpcache.cxx

@@ -161,6 +161,8 @@ void initFromCell(
     {
         rData.SetString(rCache.InternString(aDocStr));
     }
+    else
+        rData.SetEmpty();
 }
 
 void getItemValue(

sc/source/core/data/dpitemdata.cxx

@@ -138,6 +138,12 @@ ScDPItemData::Type ScDPItemData::GetType() const
     return static_cast<Type>(meType);
 }
 
+void ScDPItemData::SetEmpty()
+{
+    DisposeString();
+    meType = Empty;
+}
+
 void ScDPItemData::SetString(const rtl::OUString& rS)
 {
     DisposeString();