v5: Fix for csh exposed by bash fix

This release fixes a csh bug exposed by the bash fix in the previous release.

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From CHANGELOG.md

Fixed

• Check variable existence before echoing in g5_modules to prevent undefined variable errors when switchmodules or usemodules is queried

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What's Changed

• Fix undefined variable error in g5_modules for v5.25.2 by @mathomp4 in #192

Full Changelog: v5.25.1...v5.25.2