PR #6641 breaks text custom field default value

[bmhughes]

NetBox version

v2.11.9

Python version

3.9

Steps to Reproduce

1. Create a custom text type field.
2. Try to set a default value.

Expected Behavior

Default value should be set.

Observed Behavior

ValidationError is raised because .validate for a text field tests against str when the field is a django.forms.fields.JSONString, trying to set a str value causes a Enter a valid JSON. error.

[jeremystretch]

Linking this to #6641