[21556] Bugfix: clear map before deserializing with string as values in XCRv1

[Mario-DL]

Description

A map<T, string> was not being cleared before deserializing in XCDRv1. Test and fix included. Thanks @fschoenm for the report and @MiguelCompany for the analysis.

@Mergifyio backport 2.2.x 1.0.x

Fixes eProsima/Fast-DDS/issues/5136

Contributor Checklist

• Commit messages follow the project guidelines.
• The code follows the style guidelines of this project.
• Tests that thoroughly check the new feature have been added/Regression tests checking the bug and its fix have been added; the added tests pass locally
• N/A Any new/modified methods have been properly documented using Doxygen.
• Changes are backport compatible: they do NOT break ABI nor change library core behavior.
• Changes are API compatible.
• N/A New feature has been added to the versions.md file (if applicable).
• Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
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• Check contributor checklist is correct.
• Check CI results: changes do not issue any warning.
• Check CI results: CI pass and failing tests are unrelated with the changes.