Fix CraftMetaBanner serialization.

src/main/java/org/bukkit/craftbukkit/inventory/CraftMetaBanner.java

@@ -145,8 +145,13 @@ public class CraftMetaBanner extends CraftMetaItem implements BannerMeta {
     ImmutableMap.Builder<String, Object> serialize(ImmutableMap.Builder<String, Object> builder) {
         super.serialize(builder);
         
+        if(base != null){
             builder.put(BASE.BUKKIT, base);
+        }
+
+        if(!patterns.isEmpty()){
             builder.put(PATTERNS.BUKKIT, ImmutableList.copyOf(patterns));
+        }
 
         return builder;
     }

src/main/java/org/bukkit/craftbukkit/inventory/CraftMetaItem.java

@@ -96,6 +96,7 @@ class CraftMetaItem implements ItemMeta, Repairable {
 
         static {
             classMap = ImmutableMap.<Class<? extends CraftMetaItem>, String>builder()
+                    .put(CraftMetaBanner.class, "BANNER")
                     .put(CraftMetaBook.class, "BOOK")
                     .put(CraftMetaSkull.class, "SKULL")
                     .put(CraftMetaLeatherArmor.class, "LEATHER_ARMOR")