fix(接口测试): 修复特殊数据的swagger3.0数据导出失败的缺陷

--bug=1027526 --user=王孝刚 【接口测试】导入swagger2.0接口数据后再次导出为swagger3.0格式-导出失败
https://www.tapd.cn/55049933/s/1389001

api-test/backend/src/main/java/io/metersphere/api/parse/api/Swagger3Parser.java

@@ -1263,9 +1263,11 @@ public class Swagger3Parser extends SwaggerAbstractParser {
                 String xml = XMLUtil.delXmlHeader(xmlText);
                 int startIndex = xml.indexOf("<", 0);
                 int endIndex = xml.indexOf(">", 0);
+                if (endIndex > startIndex+ 1 ) {
                     String substring = xml.substring(startIndex + 1, endIndex);
-                JSONObject xmlToJson = XMLUtil.xmlConvertJson(xmlText);
                     bodyInfo = buildRefSchema(substring);
+                }
+                JSONObject xmlToJson = XMLUtil.xmlConvertJson(xmlText);
                 JSONObject jsonObject = buildRequestBodyXmlSchema(xmlToJson);
                 if (schemas == null) {
                     schemas = new LinkedList<>();