oss返回失败时，返回值置为空字符串

src/main/java/org/springblade/founder/flwsfj/service/impl/OssServiceImpl.java

@@ -103,7 +103,7 @@ public class OssServiceImpl implements IOssService {
 	 */
 	@Override
 	public String getOssFileStr(String filePath) {
-		String fileStr = null;
+		String fileStr = "";
 		R r = getOssFile(filePath);
 		if (ResultCode.SUCCESS.equals(r.getCode())) {
 			fileStr = r.getData().get("result").toString();