Merge pull request #45 from ucloud/fix/nullpoint

fix(common): when response is not ok, fix nullpointer exception

Author: wangrzneu

ucloud-sdk-java-common/src/main/java/cn/ucloud/common/transport/DefaultTransport.java

@@ -79,7 +79,11 @@ public Response invoke(Request request, Class<? extends Response> clazz)
         } catch (Exception e) {
             throw new TransportException("http error", e);
         }
-        String requestId = httpResponse.getLastHeader(HEADER_REQUEST_ID).getValue();
+
+        String requestId = "";
+        if (httpResponse.getLastHeader(HEADER_REQUEST_ID) != null) {
+            requestId = httpResponse.getLastHeader(HEADER_REQUEST_ID).getValue();
+        }
 
         // check http status
         StatusLine httpStatus = httpResponse.getStatusLine();