Secret: add a __bool__ implementation

[rcoup]

Allows using a Secrets in a conditional statement to check for empty/None without needing to cast it to a str.

Before:

if str(settings.A_SECRET):  # without the cast it's always True
  # do stuff

After:

if settings.A_SECRET:
  # do stuff

[Kludex]

Thanks for the PR @rcoup !