Do not fail when report is missing in reference build

[zbynek]

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• [n/a] Link to relevant issues in GitHub or Jira
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• [TODO] Ensure you have provided tests - that demonstrates feature works or fixes the issue

As an administrator I want to be able to remove the coverage .xml files for old builds without breaking new builds. Clearly this should be done using some log-rotate feature in Jenkins, but it would be nice if the plugin could handle the case when I do it by simply removing the files from the filesystem.

[codecov]

Codecov Report

Merging #232 (8e25b06) into master (81dc044) will not change coverage.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##             master     #232   +/-   ##
=========================================
  Coverage     57.98%   57.98%           
  Complexity      480      480           
=========================================
  Files            56       56           
  Lines          2304     2304           
  Branches        289      289           
=========================================
  Hits           1336     1336           
  Misses          863      863           
  Partials        105      105           

Impacted Files	Coverage Δ
...io/jenkins/plugins/coverage/CoverageProcessor.java	63.83% <0.00%> (ø)

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[zbynek]

@uhafner I didn't find any test coverage for the diff handling. Do you expect a test for this PR?

[uhafner]

No, for this change no test is required.

BTW: this will only work for the results in the old (non-xml) report serializations. The new XML based serialization is not yet used for the delta computation.