Background and border color transparency support for textboxes when color is undefined

[Chudy20007]

Description

In project we use text boxes for imprints and side notes on a word document. It's currently not possible to use non-white background with border. As a simple fix I implement simple check if border or background color is set - if not then text box should be rendered with a transparent colors, else current behavior will be used.

Fixes # (issue)

Checklist:

• I have run composer run-script check --timeout=0 and no errors were reported
• The new code is covered by unit tests (check build/coverage for coverage report)
• I have updated the documentation to describe the changes

[coveralls]

coverage: 97.218% (+0.001%) from 97.217%
when pulling 59ed844 on Chudy20007:transparent-textbox
into 2f4da6e on PHPOffice:master.