Fix#4063. Fix#1560. Fix#1293. PhpSpreadsheet is not accounting for an empty string in Xlsx sharedStrings.xml.The code which parses it in Reader/Xlsx looks for a `t` or `r` tag descending from `si`, but, in this case, the tag is coded as `<si/>`, with neither t nor r tag descending. An else clause is added to set the string to empty string in this case.
I was surprised that this had not turned up before, and a search through the archives found at least 2 earlier reports from 4 years ago. Those had been marked stale; the stale indicator is removed, and the issues are re-opened, to be closed when this PR is merged.
oleibman