updated eos_switchport trunk_allowed_vlans to not cast None

This PR is intended to resolve the following:

When I run a plain Ansible task similar to this:

```yml
  tasks:
    - name: expected to do nothing
      eos_switchport:
        name: Ethernet5
        eapi_username: "{{ eapi_username }}"
        eapi_password: "{{ eapi_password }}"
        eapi_hostname: "{{ inventory_hostname }}"
      register: results

    - debug:
        var: results.commands
```

Trunk switchports have their allowed vlans set to none, instead of taking no action.

Here is the pre-run output of `show interfaces Ethernet5 switchport`.

```
Name: Et5
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
MAC Address Learning: enabled
Access Mode VLAN: 30 (VLAN0030)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: disabled
Administrative private VLAN mapping: ALL
Trunking VLANs Enabled: 10,100
Trunk Groups:
```

And post run output:

```
Name: Et5
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
MAC Address Learning: enabled
Access Mode VLAN: 30 (VLAN0030)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: disabled
Administrative private VLAN mapping: ALL
Trunking VLANs Enabled: NONE
Trunk Groups:
```

Finally, here is part of the debug task output:

```
    "results.commands": [
        {
            "cmd": "enable",
            "input": ""
        },
        "enable",
        "configure",
        "interface Ethernet5",
        "switchport trunk allowed vlan None"
    ]
```

Let me know if there are any questions or concerns. Thanks!