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

❌ Patch coverage is 78.57143% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.93%. Comparing base (37604ef) to head (13bb6a7).
⚠️ Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/config.js	78.57%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1030      +/-   ##
==========================================
+ Coverage   80.71%   80.93%   +0.22%     
==========================================
  Files          39       39              
  Lines        4754     4841      +87     
==========================================
+ Hits         3837     3918      +81     
- Misses        917      923       +6     

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[aduh95]

Ping @nodejs/node-core-utils for reviews