RFR: 7903964: Fix NPE in JTRegConfiguration.getJDKString

[Oleksii Sylichenko]

# This bug appears when
- The user removes the "Build" target from "Before launch" in the Run Configuration.
- The user unsets or does not set the Project SDK, or if it has been removed from the file system.

# Steps to reproduce

1. Open a JDK project.
2. Open "Project Structure".
3. Select an SDK in the Project tab.
4. Navigate to "Settings → jtreg".
5. Set "JRE = "Default".
6. Run a test to automatically create a run configuration, or manually create one with "JRE = Default".
7. Open Project Structure again.
8. Set SDK = "<No SDK>".
9. Edit the previously created Run Configuration and remove the "Build" target from "Before launch".
10. Run the previously created Run Configuration.

## Expected
- Error message appeared: "No valid JDK configured for running jtreg tests"

## Actual:
- Nothing happened

# Implemented Changes

## JTRegConfiguration.java
- Add the `@NotNull` annotation to methods that are missing it but have it in the parent method.
- Add javadoc to `getJDKString`.
- Refactor `getJDKString`:
  - Fix method getJDKString, to prevent NPE when Default SDK for the project was not set

## JTRegConfigurationRunnableState.java
- Change visibility to `public` to eliminate the warning in `JTRegConfiguration#getState`.

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Commit messages:
 - JTRegConfigurationRunnableState.java:
 - JTRegConfiguration.java:

Changes: https://git.openjdk.org/jtreg/pull/251/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=251&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903964
  Stats: 31 lines in 2 files changed: 15 ins; 3 del; 13 mod
  Patch: https://git.openjdk.org/jtreg/pull/251.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/251/head:pull/251

PR: https://git.openjdk.org/jtreg/pull/251