GH-520 - Early reject invalid ApplicationModules bootstraps.

We now immediately reject the ApplicationModules bootstrap in case the initial scanning of the root packages yield no classes at all.

spring-modulith-integration-test/src/test/java/org/springframework/modulith/core/ApplicationModulesIntegrationTest.java

@@ -21,6 +21,7 @@ import example.declared.first.First;
 import example.declared.fourth.Fourth;
 import example.declared.second.Second;
 import example.declared.third.Third;
+import example.empty.EmptyApplication;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -234,6 +235,13 @@ class ApplicationModulesIntegrationTest {
 				.noneMatch(it -> it.contains("Cycle detected: Slice open"));
 	}
 
+	@Test // GH-520
+	void bootstrapsOnEmptyProject() {
+
+		assertThatNoException().isThrownBy(() -> ApplicationModules.of(EmptyApplication.class).verify());
+		assertThatIllegalArgumentException().isThrownBy(() -> ApplicationModules.of("non.existant"));
+	}
+
 	private static void verifyNamedInterfaces(NamedInterfaces interfaces, String name, Class<?>... types) {
 
 		Stream.of(types).forEach(type -> {