Fix usage of uninitialized variable

Description

The classical particles' acceleration is not always available, so we don't print it to the log anymore.

News snippet

Fix usage of uninitialized variable in classical particles.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.

changed milestone to %11.0

added Bugfix label

assigned to @heiko.appel

approved this merge request

enabled an automatic merge when the pipeline for 2e6f2d69 succeeds

Codecov Report

Merging #1090 (2e6f2d69) into develop (71aa3438) will decrease coverage by 0.00%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #1090      +/-   ##
===========================================
- Coverage    68.58%   68.58%   -0.01%     
===========================================
  Files          514      514              
  Lines        98298    98297       -1     
===========================================
- Hits         67418    67417       -1     
  Misses       30880    30880              

Impacted Files	Coverage Δ
src/classical/classical_particle.F90	88.60% <100.00%> (-0.05%)

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mentioned in commit 1076f9aa

merged