Don't add nil issuers to issuer pool

[priyawadhwa]

There's a chance the issuer can be nil if it's a custom issuer. If it's nil, don't add it to the issuer pool.

[codecov]

Codecov Report

Merging #1053 (42fdd12) into main (372b3f3) will increase coverage by 0.42%.
The diff coverage is 100.00%.

❗ Current head 42fdd12 differs from pull request most recent head 8a322b6. Consider uploading reports for the commit 8a322b6 to get more accurate results

@@            Coverage Diff             @@
##             main    #1053      +/-   ##
==========================================
+ Coverage   54.77%   55.20%   +0.42%     
==========================================
  Files          48       48              
  Lines        2521     2525       +4     
==========================================
+ Hits         1381     1394      +13     
+ Misses       1039     1032       -7     
+ Partials      101       99       -2     

Impacted Files	Coverage Δ
pkg/server/issuer_pool.go	100.00% <100.00%> (+3.33%)	⬆️

... and 2 files with indirect coverage changes

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