[AppRunner] Fixed null bug in environment conditional.

[jonryser]

Describe the changes

Tthe context.tf in 1.0.3-AppRunner had a broken validation condition that called length(null) instead of checking for null first.

Checklist before requesting a review

• My code adheres to the coding and style guidelines of the project.
• I have performed a self-review of my code.
• I have tested my changes and ensured that they work as expected.
• I have updated examples and documentation as necessary.
• I have incremented the version number in the appropriate files.