Fix backwards compatibility with 1.19.4

Author: Joo200

worldguard-bukkit/src/main/java/com/sk89q/worldguard/bukkit/util/Materials.java

@@ -53,9 +53,10 @@ public final class Materials {
     private static final Set<PotionEffectType> DAMAGE_EFFECTS = new HashSet<>();
 
     private static void putMaterialTag(Tag<Material> tag, Integer value) {
+        if (tag == null) return;
         tag.getValues().forEach(mat -> MATERIAL_FLAGS.put(mat, value));
     }
-    private static Tag<Material> SIGNS_TAG;
+    private static Tag<Material> SIGNS_TAG = Tag.SIGNS;
 
     static {
         ENTITY_ITEMS.put(EntityType.PAINTING, Material.PAINTING);