fdo#75397: Return an empty string for empty element.

This is the behavior as of 3.6.  It had changed in 4.0 and onward by
accident, and caused the bug as reported in fdo#75397.

Change-Id: Id96fea354604b3c13cbbf2d9a73223b7725c7d66

sc/source/core/tool/scmatrix.cxx

@@ -579,7 +579,7 @@ svl::SharedString ScMatrixImpl::GetString( SvNumberFormatter& rFormatter, SCSIZE
         {
             if (!maMatFlag.get<bool>(nR, nC))
                 // not an empty path.
-                break;
+                return svl::SharedString::getEmptyString();
 
             // result of empty FALSE jump path
             sal_uLong nKey = rFormatter.GetStandardFormat( NUMBERFORMAT_LOGICAL,