bug: Fix process CPU calculation if /proc/stat CPU line has less values than expected

[ClementTsang]

Description

A description of the change and what it does. If relevant (such as any change that modifies the UI), please provide screenshots of the change:

Addresses a potential case where processing would fail if there were missing values from the CPU line of /proc/stat, and allows it to successfully return.

Issue

If applicable, what issue does this address?

Closes: #636

Testing

If relevant, please state how this was tested. All changes must be tested to work:

Please also indicate which platforms were tested. All platforms directly affected by the change must be tested:

• Windows
• macOS
• Linux

Checklist

If relevant, ensure the following have been met:

• Areas your change affects have been linted using rustfmt (cargo fmt)
• The change has been tested and doesn't appear to cause any unintended breakage
• Documentation has been added/updated if needed (README.md, help menu, etc.)
• The pull request passes the provided CI pipeline
• There are no merge conflicts

[codecov-commenter]

Codecov Report

Merging #637 (453558e) into master (065ffdb) will increase coverage by 0.33%.
The diff coverage is 96.66%.

@@            Coverage Diff            @@
##           master    #637      +/-   ##
=========================================
+ Coverage    6.82%   7.15%   +0.33%     
=========================================
  Files          52      52              
  Lines        8269    8284      +15     
=========================================
+ Hits          564     593      +29     
+ Misses       7705    7691      -14     

Impacted Files	Coverage Δ
src/app/data_harvester/processes/linux.rs	20.42% <96.66%> (+20.42%)	⬆️

---

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 d6a112b...453558e. Read the comment docs.