Improve error message when empty notebook is passed

[Zethson]

Fixes #274

[github-actions]

🚀 Deployed on https://658b4e8a1eb607eecfc8b2e5--nbproject-ifta.netlify.app

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (c316156) 89.39% compared to head (24d9d84) 89.06%.

Files	Patch %	Lines
nbproject/dev/_notebook.py	33.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #275      +/-   ##
==========================================
- Coverage   89.39%   89.06%   -0.34%     
==========================================
  Files          24       24              
  Lines        1028     1033       +5     
==========================================
+ Hits          919      920       +1     
- Misses        109      113       +4     

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