Resolves: tdf#100847 also external svEmptyCell is 0 in numeric context

Regression of 243f19a1 that now
differentiates between numeric and string content of external
references' cells.

Change-Id: I7a9635357cc7651353067a38f9ac8bc295f88546

sc/source/core/tool/interpr4.cxx

@@ -2043,7 +2043,7 @@ double ScInterpreter::GetDouble()
             PopExternalSingleRef(pToken);
             if (!nGlobalError)
             {
-                if (pToken->GetType() == svDouble)
+                if (pToken->GetType() == svDouble || pToken->GetType() == svEmptyCell)
                     nVal = pToken->GetDouble();
                 else
                     nVal = ConvertStringToValue( pToken->GetString().getString());