Add null checks and document them

Author: marcphilipp

junit-platform-engine/src/main/java/org/junit/platform/engine/reporting/OutputDirectoryProvider.java

@@ -30,15 +30,15 @@
 public interface OutputDirectoryProvider {
 
 	/**
-	 * {@return the root directory for all output files}
+	 * {@return the root directory for all output files; never {@code null}}
 	 */
 	Path getRootDirectory();
 
 	/**
 	 * Create an output directory for the supplied test descriptor.
 	 *
 	 * @param testDescriptor the test descriptor for which to create an output
-	 *                       directory
+	 *                       directory; never {@code null}
 	 * @return the output directory
 	 * @throws IOException if the output directory could not be created
 	 */

junit-platform-launcher/src/main/java/org/junit/platform/launcher/core/HierarchicalOutputDirectoryProvider.java

@@ -18,6 +18,7 @@
 import java.util.function.Supplier;
 import java.util.regex.Pattern;
 
+import org.junit.platform.commons.util.Preconditions;
 import org.junit.platform.engine.TestDescriptor;
 import org.junit.platform.engine.UniqueId.Segment;
 import org.junit.platform.engine.reporting.OutputDirectoryProvider;
@@ -42,6 +43,8 @@ class HierarchicalOutputDirectoryProvider implements OutputDirectoryProvider {
 
 	@Override
 	public Path createOutputDirectory(TestDescriptor testDescriptor) throws IOException {
+		Preconditions.notNull(testDescriptor, "testDescriptor must not be null");
+
 		List<Segment> segments = testDescriptor.getUniqueId().getSegments();
 		Path relativePath = segments.stream() //
 				.skip(1) //