bug fix for no cover image

lib/tajian.mjs

@@ -45,6 +45,7 @@ class TaJian {
             let content = data.title;
             await writeFile(filepath, content, { encoding: 'utf8' });
 
+            //save cover image
             if (typeof(data.cover_base64) != 'undefined' && data.cover_base64) {
                 filepath = `${dirPath}/${filename}.${data.cover_type}`;
                 content = Buffer.from(data.cover_base64, "base64");                         //保存图片文件
@@ -53,7 +54,7 @@ class TaJian {
                 filepath = `${dirPath}/${filename}_cover.txt`;
                 content = `${filename}.${data.cover_type}`;                                 //保存图片路径
                 await writeFile(filepath, content, { encoding: 'utf8' });
-            }else {
+            }else if (typeof(data.cover) != 'undefined' && data.cover) {
                 filepath = `${dirPath}/${filename}_cover.txt`;
                 content = data.cover;                                                       //保存图片网址
                 await writeFile(filepath, content, { encoding: 'utf8' });