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

@@ -57,7 +57,9 @@ abstract class AbstractApplicationLauncher implements BeforeEachCallback, AfterE
 
 	@Override
 	public void afterEach(ExtensionContext context) throws Exception {
-		this.process.destroy();
+		if (this.process != null) {
+			this.process.destroy();
+		}
 	}
 
 	@Override