Allow default package for PackageSource

(cherry picked from commit 63362e9)

Author: marcphilipp

documentation/src/docs/asciidoc/release-notes/release-notes-5.13.4.adoc

@@ -28,6 +28,8 @@ repository on GitHub.
 
 * `ClasspathResourceSelector` no longer allows to be constructed with a resource name that
   is blank after removing the leading slash.
+* `PackageSource.from(String)` now allows to be constructed with an empty string to
+  indicate the default package.
 
 [[release-notes-5.13.4-junit-platform-deprecations-and-breaking-changes]]
 ==== Deprecations and Breaking Changes

junit-platform-engine/src/main/java/org/junit/platform/engine/support/descriptor/PackageSource.java

@@ -11,6 +11,7 @@
 package org.junit.platform.engine.support.descriptor;
 
 import static org.apiguardian.api.API.Status.STABLE;
+import static org.junit.platform.commons.util.StringUtils.isNotBlank;
 
 import java.util.Objects;
 
@@ -58,7 +59,10 @@ private PackageSource(Package javaPackage) {
 	}
 
 	private PackageSource(String packageName) {
-		this.packageName = Preconditions.notBlank(packageName, "package name must not be null or blank");
+		Preconditions.notNull(packageName, "package name must not be null");
+		Preconditions.condition(packageName.isEmpty() || isNotBlank(packageName),
+			"package name must not contain only whitespace");
+		this.packageName = packageName;
 	}
 
 	/**

platform-tests/src/test/java/org/junit/platform/engine/support/descriptor/PackageSourceTests.java

@@ -18,6 +18,8 @@
 import java.util.stream.Stream;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
 import org.junit.platform.commons.PreconditionViolationException;
 
 /**
@@ -47,9 +49,11 @@ void packageSourceFromNullPackageReference() {
 		assertThrows(PreconditionViolationException.class, () -> PackageSource.from((Package) null));
 	}
 
-	@Test
-	void packageSourceFromPackageName() {
-		var testPackage = getClass().getPackage().getName();
+	@ParameterizedTest
+	@ValueSource(classes = PackageSourceTests.class)
+	@ValueSource(strings = "DefaultPackageTestCase")
+	void packageSourceFromPackageName(Class<?> testClass) {
+		var testPackage = testClass.getPackage().getName();
 		var source = PackageSource.from(testPackage);
 
 		assertThat(source.getPackageName()).isEqualTo(testPackage);

platform-tests/src/test/java/org/junit/platform/launcher/core/DiscoveryIssueCollectorTests.java

@@ -69,6 +69,7 @@ public static Stream<Pair> pairs() {
 			new Pair(selectClasspathResource("someResource", FilePosition.from(42, 23)),
 				ClasspathResourceSource.from("someResource",
 					org.junit.platform.engine.support.descriptor.FilePosition.from(42, 23))), //
+			new Pair(selectPackage(""), PackageSource.from("")), //
 			new Pair(selectPackage("some.package"), PackageSource.from("some.package")), //
 			new Pair(selectFile("someFile"), FileSource.from(new File("someFile"))), //
 			new Pair(selectFile("someFile", FilePosition.from(42)),