Fix#4411. User copied some code from a PHPExcel program which set DisplayAsBlanks to `0`. This resulted in what Excel deemed a corrupt spreadsheet using PhpSpreadsheet. The only values allowed for that field are `gap`, `zero` (not `0`), and `span`. PHPExcel used `0` as a default, and got away with it because it ignored the value entirely when writing out the spreadsheet, using `gap` all the time.
I had to choose between throwing an exception and just using the default when an attempt is made to set that property to an invalid value. An exception just seems more punitive than helpful to me, especially if we want people to migrate from PHPExcel, which still seems to have a large user base. So I've gone with using `gap` in place of an invalid value. Note that, according to https://learn.microsoft.com/ru-ru/openspecs/office_standards/ms-oe376/b5c5c694-21d9-437c-9a4a-21e0e843eed8, `gap` is used as the default whenever it is permitted for the chart in question; and, when it isn't permitted, the chart will use its default method (which will always be `zero`).
There were no tests nor samples for this property. All the tests and samples which use it use only `gap`. I have added a small test, and a new sample to illustrate the difference between the 3 options.
oleibman