related tdf#42915, pass NaN through date transformation

Many places in chart2 use NaN to mean no available value. Not propagating
NaN through the helper disables all this functionality.

Change-Id: I37f966007b5b7cc16778c5c6903710fbd144631b
Reviewed-on: https://gerrit.libreoffice.org/64266
Tested-by: Jenkins
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>

chart2/source/view/axes/DateHelper.cxx

@@ -68,6 +68,9 @@ bool DateHelper::IsLessThanOneYearAway( const Date& rD1, const Date& rD2 )
 
 double DateHelper::RasterizeDateValue( double fValue, const Date& rNullDate, long TimeResolution )
 {
+    if (rtl::math::isNan(fValue))
+        return fValue;
+
     Date aDate(rNullDate); aDate.AddDays(::rtl::math::approxFloor(fValue));
     switch(TimeResolution)
     {