Add more nullability annotations to core/spring-boot

See gh-46587

Author: mhalbritter

core/spring-boot/src/main/java/org/springframework/boot/ApplicationContextFactory.java

@@ -76,7 +76,7 @@ public interface ApplicationContextFactory {
 	 * @param webApplicationType the web application type
 	 * @return the newly created application context
 	 */
-	ConfigurableApplicationContext create(@Nullable WebApplicationType webApplicationType);
+	@Nullable ConfigurableApplicationContext create(@Nullable WebApplicationType webApplicationType);
 
 	/**
 	 * Creates an {@code ApplicationContextFactory} that will create contexts by

core/spring-boot/src/main/java/org/springframework/boot/SpringApplication.java

@@ -569,7 +569,10 @@ protected void bindToSpringApplication(ConfigurableEnvironment environment) {
 	 * @see #setApplicationContextFactory(ApplicationContextFactory)
 	 */
 	protected ConfigurableApplicationContext createApplicationContext() {
-		return this.applicationContextFactory.create(this.properties.getWebApplicationType());
+		ConfigurableApplicationContext context = this.applicationContextFactory
+			.create(this.properties.getWebApplicationType());
+		Assert.state(context != null, "ApplicationContextFactory created null context");
+		return context;
 	}
 
 	/**