Built-in middleware should gracefully handle messages without text property

[shaydewael]

Description

Currently there is a bug where builtin middleware is failing when messages are received that don't have the text property. Optional text became possible with Block Kit messages. The code that threw this was in matchMessage() though there may be more occurrences of the framework relying on a text value: https://github.com/slackapi/bolt/blob/master/src/middleware/builtin.ts#L160

What type of issue is this? (place an x in one of the [ ])

• bug
• enhancement (feature request)
• question
• documentation related
• testing related
• discussion

Requirements (place an x in each of the [ ])

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.