Ensure schema generation is robust to nans and await describe_data call

[philippjfr]

As the title says, there was an oversight which meant await wasn't called in the describe_data utility. Also the schema generation for the DuckDBSource was not robust to nans.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 55.82%. Comparing base (c2ea8c9) to head (d767527).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lumen/ai/utils.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #724      +/-   ##
==========================================
+ Coverage   55.81%   55.82%   +0.01%     
==========================================
  Files          98       98              
  Lines       11879    11881       +2     
==========================================
+ Hits         6630     6633       +3     
+ Misses       5249     5248       -1     

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