🐛 Do not mark blank=True strings as polymorphic

[stevenbal]

This was previously resulting in things like this:

# DESCRIPTION: Key the Identity Provider uses to sign ID tokens in the case of an
# RSA sign algorithm. Should be the signing key in PEM or DER format.
# DEFAULT VALUE: ""
# REQUIRED: false
# This field can have multiple different kinds of value. All the
# alternatives are listed below and are divided by dashes. Only **one of
# them** can be commented out.
# -------------ALTERNATIVE 1-------------
# oidc_rp_idp_sign_key: null
# -------------ALTERNATIVE 2-------------
oidc_rp_idp_sign_key: example_string