fix: patch should not reset client secret

[lpedrosa]

Fixes the bug reported in #2869 by checking if the PATCH actually changes the Client Secret before updating. If the secret has not changed, we clear it as expected by the UpdateClient function, so the original secret remains untouched.

Related issue(s)

#2869

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security. vulnerability, I
  confirm that I got green light (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

[codecov]

Codecov Report

Merging #2872 (766a245) into master (7dd428e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2872   +/-   ##
=======================================
  Coverage   76.84%   76.85%           
=======================================
  Files         105      105           
  Lines        7143     7146    +3     
=======================================
+ Hits         5489     5492    +3     
  Misses       1248     1248           
  Partials      406      406           

Impacted Files	Coverage Δ
client/handler.go	62.50% <100.00%> (+1.03%)	⬆️

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[aeneasr]

Perfect!