gradle-plugin: treat SarifReportMergeTask as non-mandatory

[petertrr]

What's done:

• Root project might not have a task mergeDiktatReports

This pull request closes #1484

[codecov]

Codecov Report

Merging #1487 (6f90fb9) into master (4766835) will not change coverage.
The diff coverage is 66.66%.

@@            Coverage Diff            @@
##             master    #1487   +/-   ##
=========================================
  Coverage     82.96%   82.96%           
  Complexity     2579     2579           
=========================================
  Files           111      111           
  Lines          7712     7712           
  Branches       2114     2114           
=========================================
  Hits           6398     6398           
  Misses          418      418           
  Partials        896      896           

Flag	Coverage Δ
unittests	82.96% <66.66%> (ø)

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Impacted Files	Coverage Δ
...diktat/plugin/gradle/tasks/SarifReportMergeTask.kt	18.91% <66.66%> (ø)

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[github-actions]

JUnit Tests (macOS, EnricoMi/publish-unit-test-result-action@v1)

1 317 tests   1 302 ✔️  1m 22s ⏱️
   161 suites       15 💤
   161 files           0 ❌

Results for commit bbd05b7.

[github-actions]

JUnit Tests (Windows, EnricoMi/publish-unit-test-result-action@v1)

1 332 tests   1 317 ✔️  3m 14s ⏱️
   162 suites       15 💤
   162 files           0 ❌

Results for commit bbd05b7.