fix: nil pointer dereference on UserInfoAddress

[muhlemmer]

When the Address field in userinfo is nil, an empty instance of userInfoAddress is now returned. This allows for safe chaining of Get methods. All of an empty userInfoAddress' Get methods will return empty strings.

Fixes #203

[codecov[bot]]

Codecov Report

Merging #207 (9b63a93) into main (1535ea4) will increase coverage by 0.08%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #207      +/-   ##
==========================================
+ Coverage   39.49%   39.58%   +0.08%     
==========================================
  Files          74       74              
  Lines        5907     5912       +5     
==========================================
+ Hits         2333     2340       +7     
+ Misses       3323     3321       -2     
  Partials      251      251              

Impacted Files	Coverage Δ
pkg/oidc/userinfo.go	53.65% <100.00%> (+2.71%)	:arrow_up:

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[muhlemmer]

I've updated this PR to catch the nil in userinfo.UnmarshalJSON and set a new empty value to the userinfo.Address field. The original change in userinfo.GetAddress() is reverted. And the tests now use actual json.

[muhlemmer]

@livio-a I've rebased, because the test were failing with my old branch. Comments are resolved.