add missing nullmarked configuration to test classes

Author: rob93c

src/test/java/com/github/stickerifier/stickerify/ResourceHelper.java

@@ -2,8 +2,11 @@
 
 import static org.junit.jupiter.api.Assertions.assertNotNull;
 
+import org.jspecify.annotations.NullMarked;
+
 import java.io.File;
 
+@NullMarked
 public final class ResourceHelper {
 
 	public static File loadResource(String filename) {

src/test/java/com/github/stickerifier/stickerify/junit/package-info.java

@@ -0,0 +1,4 @@
+@NullMarked
+package com.github.stickerifier.stickerify.junit;
+
+import org.jspecify.annotations.NullMarked;