Fix TypeError when JSON table contains array instead of hash

What does this MR do and why?

Problem

Sentry error: https://new-sentry.gitlab.net/organizations/gitlab/issues/1815052

TypeError: no implicit conversion of String into Integer (TypeError)

        return unless json['markdown']
                           ^^^^^^^^^^
  from lib/banzai/filter/json_table_filter.rb:47:in `process_json_table'
  from lib/banzai/filter/json_table_filter.rb:26:in `block in call'

Solution

Check that parsed JSON is a Hash before accessing string keys to prevent TypeError when JSON contains an array.

How to set up and validate locally

1. Open any issue or merge request
2. Leave a comment with this content

3. It should be rendered as an empty table instead of an error

Before	After

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