Resolves: tdf#120238 remove wrong assert()

There may be a non-empty RowMap even if
nColCount == 0 || nRowCount == 0
if there was only one column or row (or cell) selected and
bRowHeaders and/or bColHeaders were set, thus nColCount and/or
nRowCount were decremented by 1.

Regression from

commit b1f9aa5f
CommitDate: Fri Sep 28 09:04:07 2018 +0200

    loplugin:useuniqueptr in ScChartPositionMap

Change-Id: I942e3fcc92449da57d708f654000387f862a4066

sc/source/core/tool/chartpos.cxx

@@ -406,7 +406,6 @@ void ScChartPositioner::CreatePositionMap()
     {   // create an entry without data
         RowMap& rCol = aColMap[0];
         nColCount = 1;
-        assert ( rCol.empty() );
         rCol[ 0 ] = nullptr;
         nRowCount = 1;
         nColAdd = 0;