PHPOffice/PHPExcel GH-558 Check if Styles exist in Excel2003XML file

[isleshocky77]

I have encountered an Excel2003XML file which does not contain styles at all. Due to this the reader throws a warning when trying to iterate over the styles array.

Warning: Invalid argument supplied for foreach() in /vendor/phpoffice/phpexcel/Classes/PHPExcel/Reader/Excel2003XML.php on line 411

This is:

- [X] a bugfix

Checklist:

• Changes are covered by unit tests
• 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
• Documentation is updated as necessary

Why this change is needed?

See PHPOffice/PHPExcel#558 and PHPOffice/PHPExcel#559

[PowerKiKi]

Instead of adding another nested level, please extract everything into a private method readStyles() which return early if there is no style present.

And add a unit test for that case.

[isleshocky77]

@PowerKiKi See the Issue and PR from the other project from two years ago. I can't even find the file in question at this point. I broke out the block of code as requested, but that's it.