Update export.sh so it can be sourced by bash -u

[tcv-git]

Description

Robust shell scripts set the -u option to fail on use of undefined variables.

When such a script sources export.sh, it terminates.

This change updates export.sh to correctly check for an undefined variable.

Related

N/A

Testing

Run export.sh from a bash script with the -u option set, with and without the IDF_PATH environment variable set.

Checklist

Before submitting a Pull Request, please ensure the following:

• [ x] This PR does not introduce breaking changes.
• [ x] All CI checks (GH Actions) pass.
• [ x] Documentation is updated as needed.
• [ x] Tests are updated or added as necessary.
• [ x] Code is well-commented, especially in complex areas.
• [ x] Git history is clean — commits are squashed to the minimum necessary.

[CLAassistant]

All committers have signed the CLA.