Improve checks title and content

[uhafner]

Currently, there are too many assumptions about the existing values. We need to add some guards to improve the title so that only meaningful results are actually produces.

• When there are modified lines, print coverages of those lines. Otherwise, fall back to project coverage.

Will close #580.

[codecov]

Codecov Report

Merging #587 (5078829) into master (d14733d) will increase coverage by 0.15%.
The diff coverage is 93.61%.

@@             Coverage Diff              @@
##             master     #587      +/-   ##
============================================
+ Coverage     72.16%   72.32%   +0.15%     
- Complexity     1550     1575      +25     
============================================
  Files           127      127              
  Lines          6068     6059       -9     
  Branches        633      637       +4     
============================================
+ Hits           4379     4382       +3     
+ Misses         1473     1456      -17     
- Partials        216      221       +5     

Impacted Files	Coverage Δ
...gins/coverage/metrics/steps/CoverageViewModel.java	51.58% <ø> (ø)
...overage/metrics/steps/CoverageChecksPublisher.java	90.50% <92.85%> (-1.73%)	⬇️
...ugins/coverage/metrics/model/ElementFormatter.java	56.20% <100.00%> (+4.35%)	⬆️
...ns/coverage/metrics/steps/CoverageBuildAction.java	76.36% <100.00%> (+9.07%)	⬆️

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