Fixes #20221: JSON CustomField does not coerce {} to null

This fix actually fixes this for all valid JSON values that evaluate to
`False` in Python when loaded and cast to bool:
`bool(json.loads(<val>))`.

- `{}`
- `[]`
- `0`
- `False`

This does not change the behavior of `()` or `""` which are both
explicitly cited as "empty" values on `JSONField`.

netbox/utilities/templates/builtins/customfield_value.html

@@ -14,7 +14,7 @@
   {{ value|isodatetime }}
 {% elif customfield.type == 'url' and value %}
   <a href="{{ value }}">{{ value|truncatechars:70 }}</a>
-{% elif customfield.type == 'json' and value %}
+{% elif customfield.type == 'json' and value is not None %}
   <pre>{{ value|json }}</pre>
 {% elif customfield.type == 'multiselect' and value %}
   {{ value|join:", " }}