Circuit Provider Account field required API not in GUI
### NetBox version

v3.5.0

### Python version

3.8

### Steps to Reproduce

1. Use the REST API to create a new circuit
`
{
    "cid": "12345",
    "provider": 1,
    "type": 1,
}
`
Error: {"provider_account":["This field is required."]}
2. Use the GUI to add circuit, `provider_account` is an optional field
3. Try to add circuit with provider_account set as 0
`
{
    "cid": "12345",
    "provider": 1,
    "type": 1,
    "provider_account": 0
}
`
Error: {"provider_account":["Related object not found using the provided numeric ID: 0"]}
4. Read existing migrated circuit from netbox
` ...."provider_account":null ...
` 

### Expected Behavior

The circuit `provider_account` field should be optional in the RESTAPI as it current is in the GUI.

### Observed Behavior

Error was shown requiring the provider_account field to be defined
diff --git a/netbox/circuits/api/serializers.py b/netbox/circuits/api/serializers.py
index 5635b6730..f4abda645 100644
--- a/netbox/circuits/api/serializers.py
+++ b/netbox/circuits/api/serializers.py
@@ -106,7 +106,7 @@ class CircuitCircuitTerminationSerializer(WritableNestedSerializer):
 class CircuitSerializer(NetBoxModelSerializer):
     url = serializers.HyperlinkedIdentityField(view_name='circuits-api:circuit-detail')
     provider = NestedProviderSerializer()
-    provider_account = NestedProviderAccountSerializer()
+    provider_account = NestedProviderAccountSerializer(required=False, allow_null=True)
     status = ChoiceField(choices=CircuitStatusChoices, required=False)
     type = NestedCircuitTypeSerializer()
     tenant = NestedTenantSerializer(required=False, allow_null=True)
diff --git a/netbox/circuits/forms/bulk_import.py b/netbox/circuits/forms/bulk_import.py
index d55831008..3941ef574 100644
--- a/netbox/circuits/forms/bulk_import.py
+++ b/netbox/circuits/forms/bulk_import.py
@@ -74,7 +74,8 @@ class CircuitImportForm(NetBoxModelImportForm):
     provider_account = CSVModelChoiceField(
         queryset=ProviderAccount.objects.all(),
         to_field_name='name',
-        help_text=_('Assigned provider account')
+        help_text=_('Assigned provider account'),
+        required=False
     )
     type = CSVModelChoiceField(
         queryset=CircuitType.objects.all(),
['netbox/circuits/forms/bulk_import.py', 'netbox/circuits/api/serializers.py']