[21009] Do not require PYTHON_VERSION to be defined in .bat files

[Mario-DL]

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

• [X] Commit messages follow the project guidelines.
• [X] 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)
• [X] Changes are ABI compatible.
• [X] 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.
• [X] Applicable backports have been included in the description.

Reviewer Checklist

• [x] The PR has a milestone assigned.
• [x] The title and description correctly express the PR's purpose.
• [x] 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.

[Mario-DL]

@richiprosima please test windows

[MiguelCompany]

@Mergifyio backport 2.14.x 2.13.x

[mergify[bot]]

backport 2.14.x 2.13.x

✅ Backports have been created

• #4797 [21009] Do not require PYTHON_VERSION to be defined in .bat files (backport #4770) has been created for branch 2.14.x
• #4798 [21009] Do not require PYTHON_VERSION to be defined in .bat files (backport #4770) has been created for branch 2.13.x