Protect against null description

Update `removeLineBreaks` to also work with `null`.

See gh-4703

spring-boot-tools/spring-boot-maven-plugin/src/main/java/org/springframework/boot/maven/RepackageMojo.java

@@ -268,7 +268,7 @@ public class RepackageMojo extends AbstractDependencyFilterMojo {
 	}
 
 	private String removeLineBreaks(String description) {
-		return description.replaceAll("\\s+", " ");
+		return (description == null ? null : description.replaceAll("\\s+", " "));
 	}
 
 	private void putIfMissing(Properties properties, String key,