Fix potential NPE with unknown Spring Boot starter

Fixes gh-41

initializr/src/main/groovy/io/spring/initializr/InitializrMetadata.groovy

@@ -223,6 +223,7 @@ class InitializrMetadata {
 		def asSpringBootStarter(String name) {
 			groupId = 'org.springframework.boot'
 			artifactId = StringUtils.hasText(name) ? 'spring-boot-starter-' + name : 'spring-boot-starter'
+			id = name
 		}
 
 		/**

initializr/src/test/groovy/io/spring/initializr/web/MainControllerIntegrationTests.groovy

@@ -144,6 +144,11 @@ class MainControllerIntegrationTests extends AbstractInitializrControllerIntegra
 		slurper.parseText(restTemplate.getForObject(createUrl('/metrics'), String))
 	}
 
+	@Test
+	void downloadWithUnknownSpringBootStarter() { // Simple id are accepted as spring-boot-starter
+		 downloadZip('/starter.zip?style=foo').pomAssert().hasSpringBootStarterDependency('foo')
+	}
+
 	// Existing tests for backward compatibility
 
 	@Test