Default empty value for variable not defined

[BarbUk]

Description

Fix unset variable by using a default empty value.

Motivation and Context

Using set -eu gave error for variable test with test -n if the variable does not exists.

How Has This Been Tested?

Running the theme with set -eu

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• [C] If my change requires a change to the documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh. (CF: Lint files #2378)
• I have added tests to cover my changes, and all the new and existing tests pass.

[BarbUk]

#2378 should be merged before for the CI to pass.

[BarbUk]

Thanks :)