[#24288] Clarify default empty Partition behavior in Partition QoS

[Danipiza]

Description

This PR clarifies how the default partition works in DDS Partition QoS.

Changes:

• Explicitly states that the default partition name is the empty string ("").
• Clarifies that {} is equivalent to {""}.
• Keeps tables aligned with the empty/default partition semantics.

Contributor Checklist

• Commit messages follow the project guidelines.
• Code snippets related to the added documentation have been provided.
• Documentation tests pass locally.
• The Pro version badge has been added if the documented feature is exclusive to Fast DDS Pro.
• Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.
• CI passes without warnings or errors.

[zesk1999]

LGTM with green CI