This commit checks for null values for keys within "attachment" in
the Slack integration's incoming payloads. These keys were expected
to exist optionally previously, and the existence of null values for
these wasn't anticipated. Due to an issue report for such null
values in the payload, their handling is updated appropriately.
The checks for these values are truthiness checks since the strategy
for these values being null or falsy ("", 0) is the same; we don't
process that key-value pair. This is consistent with how Slack handles
this scenario.
For the case where all the attachment fields have null values, Slack
displays this as an empty block with no content, and therefore our
strategy for this is a no-op.
Tests updated.
This adds a webhook that can be used to interpret standard Slack
payloads. Since there are a ton of existing Slack integrations out
there, having a webhook which can accept standard Slack payloads can
significantly ease transition pains. Obviously this can't do everything
that Slack payloads can (particularly WRT their widgets/interactions),
but we can ingest text and parse out multi-block payloads into a message
relatively reasonably.