Protect against NPE in server tests

Update `AbstractApplicationLauncher` to not attempt to shutdown
the process if startup fails.

spring-boot-tests/spring-boot-integration-tests/spring-boot-server-tests/src/test/java/org/springframework/boot/context/embedded/AbstractApplicationLauncher.java

@@ -57,8 +57,10 @@ abstract class AbstractApplicationLauncher implements BeforeEachCallback, AfterE
 
 	@Override
 	public void afterEach(ExtensionContext context) throws Exception {
+		if (this.process != null) {
 			this.process.destroy();
 		}
+	}
 
 	@Override
 	public void beforeEach(ExtensionContext context) throws Exception {