add check before iterating over spec templates

[patrix58]

Fixes an issue encountered while running a Nebari v2024.5.1 on Kubernetes v1.23

Reference Issues or PRs

What does this implement/fix?

Put a x in the boxes that apply

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds a feature)
• Breaking change (fix or feature that would cause existing features not to work as expected)
• Documentation Update
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no API changes)
• Build related changes
• Other (please describe):

Testing

• Did you test the pull request locally?
• Did you add new tests?

We tested it on our cloud with building a custom image on our docker registry and referencing that image in the Nebari deployment configuration.
We plan to reference the official image again in the future as soon as this change is being released.

Documentation

Not documented, non breaking change.