Handle Empty String in SharedStrings

[oleibman]

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.

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.