[BUG] Code coverage report can't be generated properly due to defect in code-coverage.gradle

[tlfeng]

Describe the bug
There are 2 bugs in code-coverage.gradle:

1. When running ./gradlew check -Dtests.coverage=true, the following error occurs:

Suite: Test class org.opensearch.test.loggerusage.OpenSearchLoggerUsageTests
  2> java.lang.RuntimeException: unable to install test security manager
        at org.opensearch.bootstrap.BootstrapForTesting.<clinit>(BootstrapForTesting.java:190)
        at org.opensearch.test.OpenSearchTestCase.<clinit>(OpenSearchTestCase.java:251)
        at java.base/java.lang.Class.forName0(Native Method)
        at java.base/java.lang.Class.forName(Class.java:468)
        at com.carrotsearch.randomizedtesting.RandomizedRunner$2.run(RandomizedRunner.java:623)

        Caused by:
        java.lang.NullPointerException: Cannot invoke "String.length()" because "input" is null
...

2. Whether the system property "tests.coverage.html_report" is set or not, the html format code coverage report is always generated.

Reason:

1. I used the same System property "tests.coverage" from an old code into my gradle script.
2. html format code coverage report is generated by default by JaCoCo gralde plugin, see here

To Reproduce
Steps to reproduce the behavior:
For bug 1: run ./gradlew check -Dtests.coverage=true
For bug 2: run ./gradlew codeCoverageReportForUnitTest or ./gradlew codeCoverageReport, without having -Dtests.coverage.html_report, the code coverage report in html format is generated in /build/reports/jacoco/

Expected behavior

1. ./gradlew check -Dtests.coverage=true can run gradle check and generate code coverage report without error.
2. Without having tests.coverage.html_report property, html format shouldn't be generated, and only xml format is generated. Only when adding the system property tests.coverage.html_report, such as running ./gradlew codeCoverageReport -Dtests.coverage.html_report, the report in html format can be generated.

Plugins
Please list all plugins currently enabled.

Screenshots
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Host/Environment (please complete the following information):

• OS: [e.g. iOS]
• Version [e.g. 22]

Additional context
Imported by #971
Related issue: #850