fix: fix empty mixin config error

Author: MC-XiaoHei

java21/src/main/java/org/leavesmc/leavesclip/mixin/LeavesPluginMeta.java

@@ -66,5 +66,9 @@ public String getAccessWidener() {
         public void setAccessWidener(String accessWidener) {
             this.accessWidener = accessWidener;
         }
+
+        public boolean isValid() {
+            return packageName != null;
+        }
     }
 }

java21/src/main/java/org/leavesmc/leavesclip/mixin/PluginResolver.java

@@ -172,7 +172,7 @@ private static boolean isOutdatedMixinJar(@NotNull File jar) {
         JarFile jarFile = entry.second();
         LeavesPluginMeta pluginMeta = entry.third();
 
-        if (pluginMeta.getMixin() == null) {
+        if (pluginMeta.getMixin() == null || !pluginMeta.getMixin().isValid()) {
             return null;
         }