[21009] Do not require PYTHON_VERSION to be defined in .bat files (backport #4770)

[mergify]

Description

@MiguelCompany identified an error introduced in #4759 as the PYTHON_VERSION does not necessarily need to be defined when executing the fastdds.bat or ros-discovery.bat scripts.
The change simply requires a specific version of Python only if PYTHON_VERSION is defined.

Tested on Windows.

@Mergifyio backport 2.14.x 2.13.x

Cherry-pick on #4765 and #4766

Contributor Checklist

• Commit messages follow the project guidelines.

• The code follows the style guidelines of this project.

• N/A 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.

• N/A Any new configuration API has an equivalent XML API (with the corresponding XSD extension)

• Changes are ABI compatible.

• Changes are API compatible.

• N/A New feature has been added to the versions.md file (if applicable).

• N/A New feature has been documented/Current behavior is correctly described in the documentation.

• 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.
• N/A Check CI results: changes do not issue any warning.
• N/A Check CI results: failing tests are unrelated with the changes.

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This is an automatic backport of pull request #4770 done by [Mergify](https://mergify.com).