SET_CELL_FORMULA: set formula even if cell was uninitialized (#868)

* Fix #866

Co-authored-by: Abo <andrea@borgia.bo.it>
Co-authored-by: sandraros <sandra.rossi@gmail.com>
Co-authored-by: Lars Hvam <larshp@hotmail.com>

src/zcl_excel_worksheet.clas.abap

@@ -3724,8 +3724,8 @@ CLASS zcl_excel_worksheet IMPLEMENTATION.
     lv_column = zcl_excel_common=>convert_column2int( ip_column ).
     READ TABLE me->sheet_content ASSIGNING <sheet_content> WITH TABLE KEY cell_row    = ip_row
                                                                           cell_column = lv_column.
-    IF sy-subrc <> 0.                                                                           " Create new entry in sheet_content if necessary
-      CHECK ip_formula IS INITIAL.                                                              " no need to create new entry in sheet_content when no formula is passed
+    IF sy-subrc <> 0.                   " Create new entry in sheet_content if necessary
+      CHECK ip_formula IS NOT INITIAL.  " only create new entry in sheet_content when a formula is passed
       ls_sheet_content-cell_row    = ip_row.
       ls_sheet_content-cell_column = lv_column.
       INSERT ls_sheet_content INTO TABLE me->sheet_content ASSIGNING <sheet_content>.