Avoid NULL in String Function Call

[oleibman]

The overall problem is described in issue #3613. This PR represents only a partial solution. Very complicated formulas are resulting in calls to string functions using null arguments, which is deprecated in recent Php releases. In fact, there are many such deprecations for the spreadsheet in question. Eliminating the deprecations is easy. However, the result of the calculation is, for many cells, 0 rather than what Excel determines it should be. This can be overlooked to a certain extent, because Excel will recalculate when the spreadsheet is opened, so loading and saving the spreadsheet in question will result in a spreadsheet which looks okay when it is opened in Excel. Resolving the incorrect calculation in PhpSpreadsheet would be nice, so I'm leaving the issue open, but that looks too complicated for me to get a toehold.

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.