fix: 仪表板背景由颜色切换为图片，未加图片时背景颜色依然存在问题

frontend/src/views/panel/edit/index.vue

@@ -344,7 +344,7 @@ export default {
     mobileCanvasStyle() {
       let style
       if (this.canvasStyleData.openCommonStyle) {
-        if (this.canvasStyleData.panel.backgroundType === 'image' && this.canvasStyleData.panel.imageUrl) {
+        if (this.canvasStyleData.panel.backgroundType === 'image' && typeof (this.canvasStyleData.panel.imageUrl) === 'string') {
           style = {
             background: `url(${this.canvasStyleData.panel.imageUrl}) no-repeat`
           }