Fix banner NPE when when using the default package

Update ResourceBanner to protect against a null application package.

Fixes gh-2143

spring-boot/src/main/java/org/springframework/boot/ResourceBanner.java

@@ -99,8 +99,8 @@ public class ResourceBanner implements Banner {
 	}
 
 	protected String getApplicationVersion(Class<?> sourceClass) {
-		return (sourceClass == null ? null : sourceClass.getPackage()
-				.getImplementationVersion());
+		Package sourcePackage = (sourceClass == null ? null : sourceClass.getPackage());
+		return (sourcePackage == null ? null : sourcePackage.getImplementationVersion());
 	}
 
 	protected String getBootVersion() {