Theme File Missing But Referenced in Spreadsheet

[oleibman]

Fix #3770. A rels file points to a non-existent theme file in the spreadsheet. In other similar cases (e.g. PR #3771), Excel opens such a spreadsheet, but with an error pop-up. Not so with this file; it just opens the spreadsheet without the pop-up. PhpSpreadsheet will now account for this unusual situation as well.

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.