config: remove possibility of nothing return from get_config

[aviatesk]

Our current configuration system uses Configuration.Maybe to distinguish between "no configuration set" and "user has set some configuration", essentially allowing each configuration to be represented as nothing. As a result, get_config is currently set up to potentially return nothing, but call sites throughout the codebase do not handle cases where nothing is returned.

get_config is fundamentally used to retrieve some configuration value regardless of whether it is a default or not, and there is no need to return nothing in the first place.

This commit fixes get_config to never return nothing in all cases using DEFAULT_CONFIG, resolving this logic error and type instability.

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 69.17%. Comparing base (32021a6) to head (0460075).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/types.jl	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #302      +/-   ##
==========================================
+ Coverage   69.14%   69.17%   +0.03%     
==========================================
  Files          39       39              
  Lines        3915     3916       +1     
==========================================
+ Hits         2707     2709       +2     
+ Misses       1208     1207       -1     

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