WireGuardPeer presharedkey field needs allow_empty property

[jaredhendrickson13]

Describe the bug

The presharedkey field defaults to an empty string but the allow_empty property is not set. The allow_null property IS set but null values inherit the default value as designed.

To Reproduce

Make a POST request to /api/v2/vpn/wireguard/peer with presharedkey set to null or an empty string. You will get a 'value cannot be empty' error

Expected behavior

Empty strings should be allowed by the Model

Screenshots or Response
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pfSense Version & Package Version:

• pfSense Version: All
• Package Version v2.0.3

Affected Endpoints:

• URL: /api/v2/vpn/wireguard/peer

Additional context

Discussed in #534