Read empty vmerge

[kernusr]

Description

In MS Word 2021 docx files save with empty w:val of w:vMerge when expected value "continue".

I made suggest a solution:
In method readStyleDefs for argument styleDefs add fifth parameter "default value". This will allow default values to be passed for empty attributes. For example in a cells

The problem is described here: #2330

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

[Progi1984]

Thanks @kernusr for your contribution.