update csrf key generation

[wy65701436]

Fixes #21060

Do not generate a random key if the provided key has an invalid length.

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Comprehensive Summary of your change

Issue being fixed

Fixes #(issue)

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.09%. Comparing base (c8c11b4) to head (3c85241).
Report is 329 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #21154       +/-   ##
===========================================
+ Coverage   45.36%   66.09%   +20.72%     
===========================================
  Files         244     1049      +805     
  Lines       13333   114654   +101321     
  Branches     2719     2867      +148     
===========================================
+ Hits         6049    75775    +69726     
- Misses       6983    34736    +27753     
- Partials      301     4143     +3842     

Flag	Coverage Δ
unittests	66.09% <100.00%> (+20.72%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/server/middleware/csrf/csrf.go	86.00% <100.00%> (ø)

... and 1287 files with indirect coverage changes

[reasonerjt]

We should make document this as a break change.

[MinerYang]

lgtm