[gradle-plugin] use empty string as default value for reporter type

[petertrr]

In #1426 we've introduced a warning, that is currently being logged in unintended cases, because default value of reporterType is a non-empty string:

`diktat.githubActions` is set to true, so custom reporter [plain] will be ignored and SARIF reporter will be used

However, reporter type is not set explicitly so for the end user the warning makes no sense.

With an empty string as the default there will be no warning and diktat will fall back to using plain reporter according to the DiktatJavaExecTaskBase#setReporter method

[github-actions]

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

1 286 tests   1 270 ✔️  1m 1s ⏱️
   158 suites       16 💤
   158 files           0 ❌

Results for commit 67ca69c.

♻️ This comment has been updated with latest results.

[github-actions]

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

1 300 tests   1 285 ✔️  2m 31s ⏱️
   159 suites       15 💤
   159 files           0 ❌

Results for commit 67ca69c.

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

Merging #1458 (67ca69c) into master (7582190) will decrease coverage by 0.61%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1458      +/-   ##
============================================
- Coverage     83.23%   82.62%   -0.62%     
+ Complexity     2562     2542      -20     
============================================
  Files           107      104       -3     
  Lines          7619     7471     -148     
  Branches       2103     2086      -17     
============================================
- Hits           6342     6173     -169     
- Misses          388      435      +47     
+ Partials        889      863      -26     

Flag	Coverage Δ
unittests	82.62% <ø> (-0.62%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...at/ruleset/rules/chapter3/files/IndentationRule.kt	78.92% <ø> (-0.09%)	⬇️
.../kotlin/org/cqfn/diktat/plugin/maven/DiktatMojo.kt	9.09% <0.00%> (-81.82%)	⬇️
...lin/org/cqfn/diktat/plugin/maven/DiktatBaseMojo.kt	9.57% <0.00%> (-43.62%)	⬇️
...n/org/cqfn/diktat/plugin/gradle/DiktatExtension.kt
...qfn/diktat/plugin/gradle/DiktatJavaExecTaskBase.kt
...rg/cqfn/diktat/plugin/gradle/DiktatGradlePlugin.kt

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