Added condition for link response in `download.Fluxnet2015`

[meetagrawal09]

Description

This adds an additional condition when checking for dataURLsList in link object. Made this improvement as it does not check conditions when the link is NULL or when the return value is atomic.

Work done : I ran the API code separately and ended up in the result as below in the link variable :

  [1] "This endpoint is no longer available. Please contact ameriflux-support@lbl.gov for assistance."

which is atomic and thus the condition seemed necessary.

sitename set to : Niwot Ridge Forest/LTER NWT1

Review Time Estimate

• [ ] Immediately
• [ ] Within one week
• [X] When possible

Types of changes

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] My name is in the list of CITATION.cff
• [ ] I have updated the CHANGELOG.md.
• [ ] I have updated the documentation accordingly.
• [X] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[meetagrawal09]

I think that working on correcting the endpoint can be another unit of work for completely fixing this.