Return nil if meta_description or need_id are missing from a flow

second half of: https://www.agileplannerapp.com/boards/105200/cards/6367
Wondering if we could go slightly simpler here and avoid reimplementing the #fetch default by doing something like:
```
def fetch(*args)
  args[0] = args[0].to_s
  @attributes.fetch(*args)
end
```
?