Improve null-safety of module/spring-boot-mustache

See gh-46926

module/spring-boot-mustache/src/main/java/org/springframework/boot/mustache/autoconfigure/MustacheProperties.java

@@ -195,13 +195,13 @@ public class MustacheProperties {
 		 */
 		private boolean exposeSpringMacroHelpers = true;
 
-		private final Supplier<Charset> charset;
+		private final Supplier<@Nullable Charset> charset;
 
 		public Servlet() {
 			this.charset = () -> null;
 		}
 
-		private Servlet(Supplier<Charset> charset) {
+		private Servlet(Supplier<@Nullable Charset> charset) {
 			this.charset = charset;
 		}