fix: Readme descriptions that say undefined because they're empty

[nalanj]

Describe your changes

Fixes up our readme generator to not show anything after description if it's unset in the nango.yaml. Also adds a warning so they're easier to spot.

Issue ticket number and link

Checklist before requesting a review (skip if just adding/editing APIs & templates)

• I added tests, otherwise the reason is:
• External API requests have retries
• Pagination is used where appropriate
• The built in nango.paginate call is used instead of a while (true) loop
• Third party requests are NOT parallelized (this can cause issues with rate limits)
• If a sync requires metadata the nango.yaml has auto_start: false
• If the sync is a full sync then track_deletes: true is set
• I followed the best practices and guidelines from the Writing Integration Scripts doc