#390: add hasPrefix to xmlnode / xmlattributes

[fbergmann]

Description

This PR adds a _hasPrefix method to xmlnode and xmlattributes. It also fixes the documentation string,
to make clear that indeed NULL will be returned by _getPrefix if no prefix is present.

Motivation and Context

fixes #390

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Change in documentation

Checklist:

• I have updated all documentation necessary.
• I have checked spelling in (new) comments.

Testing

• Testing is done automatically and codecov shows test coverage
• This cannot be tested automatically