fix(ncu-config): allow overriding global encrypted keys with local ones

[aduh95]

If a value is defined locally, it should make no difference whether the associated key is present or not in the global config.

I hit that as I tried setting a temp GH token to access the private repo for the security release, and got an error sent from the encrypted value setter, which gets reached because Object.assign calls it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.74%. Comparing base (37604ef) to head (145e79b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1030      +/-   ##
==========================================
+ Coverage   80.71%   80.74%   +0.03%     
==========================================
  Files          39       39              
  Lines        4754     4762       +8     
==========================================
+ Hits         3837     3845       +8     
  Misses        917      917              

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