Device List Role filter duplicates first item

[stuntguy3000]

Environment

• Python version: 2.7.5
• NetBox version: v2.5.7

Steps to Reproduce

1. Go to Devices overview page
2. Try to filter by Role

Expected Behavior

The device roles should appear only once.

Observed Behavior

[zelfix]

There is the same bug

[jeremystretch]

@lampwins think you'd be able to take a look at this?

[lampwins]

Yes, I will take care of this.

[lampwins]

For reference, this occurs when null_option is True on the widget but the field does not specify a null_label.

    role = FilterChoiceField(
        queryset=DeviceRole.objects.all(),
        to_field_name='slug',
        widget=APISelectMultiple(
            api_url="/api/dcim/device-roles/",
            value_field="slug",
            null_option=True
        )
    )