fix: Maybe uninit warning in KDTree

[paulgessinger]

This popped up for me on GCC 8. Trivial fix.

[codecov]

Codecov Report

Merging #1162 (a6f335f) into main (eb51e5b) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #1162      +/-   ##
==========================================
- Coverage   47.90%   47.90%   -0.01%     
==========================================
  Files         359      359              
  Lines       18491    18492       +1     
  Branches     8723     8724       +1     
==========================================
  Hits         8859     8859              
  Misses       3603     3603              
- Partials     6029     6030       +1     

Impacted Files	Coverage Δ
Core/include/Acts/Utilities/KDTree.hpp	55.83% <0.00%> (-0.47%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update eb51e5b...a6f335f. Read the comment docs.