fix(canvas): devicePixelRatio 类型可能为null导致app平台编译失败的问题

pages/component/canvas/canvas.uvue

@@ -14,7 +14,7 @@
 <script>
   function hidpi(canvas : UniCanvasElement) {
     const context = canvas.getContext("2d")!;
-    const dpr = uni.getDeviceInfo().devicePixelRatio;
+    const dpr = uni.getDeviceInfo().devicePixelRatio ?? 1;
     canvas.width = canvas.offsetWidth * dpr;
     canvas.height = canvas.offsetHeight * dpr;
     context.scale(dpr, dpr);