don't generate examples for jobs with no expression

[josephjclark]

Short Description

A small tweak to filter out docs examples which don't have code

For example, we should not be showing this example: https://docs.openfn.org/adaptors/library/jobs/auto/Registration-forms-new-case-2023-06-22

This PR filters stuff like this out

AI Usage

Please disclose how you've used AI in this work (it's cool, we just want to
know!):

• Code generation (copilot but not intellisense)
• Learning or fact checking
• Strategy / design
• Optimisation / refactoring
• Translation / spellchecking / doc gen
• Other
• I have not used AI

You can read more details in our
Responsible AI Policy