Support a full path in DiktatReporter

[nulls]

What's done:

• supported sourceRootDir as null in DiktatReporter
• pass sourceRootDir as null in gradle plugin

It closes #1795

[github-actions[bot]]

JUnit Tests (macOS, EnricoMi/publish-unit-test-result-action@v2)

   165 files  ±0     165 suites  ±0   6m 22s :stopwatch: - 4m 22s 1 395 tests ±0  1 359 :heavy_check_mark: ±0  36 :zzz: ±0  0 :x: ±0  2 774 runs  ±0  2 738 :heavy_check_mark: ±0  36 :zzz: ±0  0 :x: ±0 

Results for commit 69e30d9f. ± Comparison against base commit 303f0064.

:recycle: This comment has been updated with latest results.

[github-actions[bot]]

JUnit Tests (Windows, EnricoMi/publish-unit-test-result-action@v2)

   165 files  ±0     165 suites  ±0   6m 59s :stopwatch: - 6m 7s 1 395 tests ±0  1 378 :heavy_check_mark: ±0  17 :zzz: ±0  0 :x: ±0  2 774 runs  ±0  2 757 :heavy_check_mark: ±0  17 :zzz: ±0  0 :x: ±0 

Results for commit a6d2499e. ± Comparison against base commit 7b7d2b6f.

:recycle: This comment has been updated with latest results.

[codecov[bot]]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (303f006) 78.41% compared to head (69e30d9) 78.38%.

Files	Patch %	Lines
...ourtool/diktat/ktlint/DiktatBaselineFactoryImpl.kt	0.00%	5 Missing :warning:
...ourtool/diktat/ktlint/DiktatReporterFactoryImpl.kt	0.00%	2 Missing :warning:
...om/saveourtool/diktat/ktlint/DiktatReporterImpl.kt	0.00%	2 Missing :warning:
...otlin/com/saveourtool/diktat/ktlint/KtLintUtils.kt	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1798 +/- ## ============================================ - Coverage 78.41% 78.38% -0.04% Complexity 2420 2420 ============================================ Files 126 126 Lines 8466 8470 +4 Branches 2147 2149 +2 ============================================ Hits 6639 6639 - Misses 867 871 +4 Partials 960 960 ```

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