Fix clang-format-check workflow file

[rafaelroquetto]

When there were no modified files, grep was returning 1 rather than 0, causing the workflow to fail.

I tested this on a separate draft PR with the following scenarios, and it appears to work:

• no modified files
• modified non .c files
• modified .c files (format compliant)
• modified .c files (non-compilant)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.24%. Comparing base (f71ed30) to head (9c59e5b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1179      +/-   ##
==========================================
+ Coverage   81.17%   81.24%   +0.06%     
==========================================
  Files         136      136              
  Lines       11447    11447              
==========================================
+ Hits         9292     9300       +8     
+ Misses       1629     1624       -5     
+ Partials      526      523       -3     

Flag	Coverage Δ
integration-test	57.03% <ø> (-0.01%)	⬇️
k8s-integration-test	58.90% <ø> (+0.08%)	⬆️
oats-test	35.81% <ø> (ø)
unittests	53.52% <ø> (-0.06%)	⬇️

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