(Webex) Remove files key with empty string value

[viveksyngh]

This commit removes sending files key with empty string value when
there are no files while sending a message. files key with empty string is causing API to fail
with below BadRequest: "Unable to retrieve content.". I have validated
that by sending request to webex api from webex developer console.

Signed-off-by: Vivek Singh vivekkmr45@yahoo.in

[msftclas]

All CLA requirements met.

[viveksyngh]

I was trying webex adapter with botbuilder and I was able to get my bot app running and getting webhook callback from webex but it was not able to send the messages. It was failing with below error.

BadRequest: Unable to retrieve content.
POST https://hydra-a.wbx2.com/v1/messages
WEBEX_TRACKING_ID: webex-js-sdk_b3d434d5-da91-48ac-bf92-99c05764affc_8

After some investigation, I found that it was sending files key with empty even though there were no files. I tried the same payload in the webex developer console and there also it failed with same error. After I removed the files with empty value, it was able to send messages.

So It will be good idea to remove this key and send only if there are files.

[benbrown]

Included in the next release -> #1929