Do not display empty NFT attributes

Description and Related Issue(s)

Fixes #2337

[x] I have tested these changes locally.
[x] I added tests to cover any new functionality, following this guide
[x] Whenever I fix a bug, I include a regression test to ensure that the bug does not reappear silently.
[x] If I have added, changed, renamed, or removed an environment variable
- I updated the list of environment variables in the documentation
- I made the necessary changes to the validator script according to the guide
- I added "ENVs" label to this pull request