修复切换子命令时，参数值被错误置为null的bug

src/components/composer/MultiParams.vue

@@ -98,6 +98,8 @@ export default defineComponent({
 
       // 使用新选择的函数独有配置的默认值
       this.getSelectSubCommand(value)?.config?.forEach((config, index) => {
+        // 有默认值时才更新
+        if (!config.defaultValue) return;
         newArgvs[this.commonConfig.length + index] = config.defaultValue;
       });
 

src/components/composer/common/VariableInput.vue

@@ -205,7 +205,7 @@ export default defineComponent({
   computed: {
     isString: {
       get() {
-        return this.modelValue.isString;
+        return this.modelValue?.isString;
       },
       set(value) {
         this.$emit("update:modelValue", {
@@ -225,7 +225,7 @@ export default defineComponent({
 
     inputValue: {
       get() {
-        return this.modelValue.value;
+        return this.modelValue?.value;
       },
       set(value) {
         this.$emit("update:modelValue", {