Recognize when loaded_at_field is explicitly set to null

[emmyoop]

resolves #9320

Problem

When loaded_at_field is explicitly set to null at the table level, it does not override the source level setting.

Solution

Add logic to recognize when the field is set vs unset.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX
• This PR includes type annotations for new and modified functions

[codecov]

Codecov Report

Attention: Patch coverage is 96.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 88.17%. Comparing base (bcbde3a) to head (797bee6).
Report is 10 commits behind head on main.

Files	Patch %	Lines
core/dbt/contracts/graph/unparsed.py	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10065      +/-   ##
==========================================
- Coverage   88.22%   88.17%   -0.06%     
==========================================
  Files         181      181              
  Lines       22643    22722      +79     
==========================================
+ Hits        19976    20034      +58     
- Misses       2667     2688      +21     

Flag	Coverage Δ
integration	85.48% <96.00%> (-0.10%)	⬇️
unit	62.37% <80.00%> (+0.06%)	⬆️

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[gshank]

Looks good!