Switch to a RuntimeException as NPE has a message on Java 15

Closes gh-22136

spring-boot-project/spring-boot-cli/src/test/java/org/springframework/boot/cli/command/CommandRunnerTests.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -141,7 +141,7 @@ class CommandRunnerTests {
 
 	@Test
 	void handlesRegularExceptionWithoutMessage() throws Exception {
-		willThrow(new NullPointerException()).given(this.regularCommand).run();
+		willThrow(new RuntimeException()).given(this.regularCommand).run();
 		int status = this.commandRunner.runAndHandleErrors("command");
 		assertThat(status).isEqualTo(1);
 		assertThat(this.calls).containsOnly(Call.ERROR_MESSAGE, Call.PRINT_STACK_TRACE);