fix(slack): slack image rendered requires text in title

[tomkcey]

The documentation makes no mention of the title property, but the error message as copy/pasted in this issue (#182) clearly states the text property is undefined. There was a null-assertion, so I removed it and added a fallback.

[linear]

DEV-1864 [BUG] Slack images will not work if you don`t provide a title (botpress/messaging #182)

Botpress core version: 12.26.3

All though titles are not required for other channels when using the image content-type, if you don`t provide, it will not work in the Slack Channel and an error will be thrown:

[ERROR] web-api:WebClient:0 failed to match all allowed schemas [json-pointer:/blocks/0/title]
[ERROR] web-api:WebClient:0 missing required field: text [json-pointer:/blocks/0/title]
09/28/2021 14:20:49.466 [Messaging] app:slack Error occurred when sending a message. Error: An API error occurred: invalid_blocks
at Object.platformErrorFromResult (C:\snapshot\messaging\node_modules@slack\web-api\dist\errors.js:51:33)
at WebClient.apiCall (C:\snapshot\messaging\node_modules@slack\web-api\dist\WebClient.js:156:28)
at processTicksAndRejections (internal/process/task_queues.js:97:5)
at async SlackCommonSender.send (C:\snapshot\messaging\packages\server\dist\channels\slack\senders\common.js)
at async SlackConduit.sendToEndpoint (C:\snapshot\messaging\packages\server\dist\channels\base\conduit.js)
at async InstanceService.sendMessageToInstance (C:\snapshot\messaging\packages\server\dist\instances\service.js)
{

to fix this you need to specify a title:

Since titles are not required by the content type, I believe the renderer should auto-generate one.

botpress/messaging #182 by @ davidvitora

[laurentlp]

Tested and looks fine to me!!