allows using empty strings in the SLD multi-subs matrix parameter

[CBiasuzzi]

Please check if the PR fulfills these requirements

• [ ] The commit message follows our guidelines
• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem?

Fixes #715

What kind of change does this PR introduce?

What is the current behavior?

What is the new behavior (if this is a feature change)?

Does this PR introduce a breaking change or deprecate an API?

• [ ] Yes
• [ ] No

If yes, please check if the following requirements are fulfilled

• [ ] The Breaking Change or Deprecated label has been added
• [ ] The migration steps are described in the following section

What changes might users need to make in their application due to this PR? (migration steps)

Other information: