Model: Allow creating an interface with empty name

But don't allow such a tunnel to be added to the tunnel manager.

(We'll need to create an interface with an empty name when we're reading QR codes.)

WireGuard/WireGuard/Model/Configuration.swift

@@ -28,7 +28,6 @@ struct InterfaceConfiguration: Codable {
     init(name: String, privateKey: Data) {
         self.name = name
         self.privateKey = privateKey
-        if (name.isEmpty) { fatalError("Empty name") }
         if (privateKey.count != 32) { fatalError("Invalid private key") }
     }
 }

WireGuard/WireGuard/UI/TunnelViewModel.swift

@@ -98,7 +98,7 @@ class TunnelViewModel {
 
         func save() -> SaveResult<InterfaceConfiguration> {
             fieldsWithError.removeAll()
-            guard let name = scratchpad[.name] else {
+            guard let name = scratchpad[.name], (!name.isEmpty) else {
                 fieldsWithError.insert(.name)
                 return .error("Interface name is required")
             }

WireGuard/WireGuard/VPN/TunnelsManager.swift

@@ -53,6 +53,7 @@ class TunnelsManager {
     }
 
     func add(tunnelConfiguration: TunnelConfiguration, completionHandler: @escaping (TunnelContainer, Error?) -> Void) {
+        assert(!tunnelConfiguration.interface.name.isEmpty)
         let tunnelProvider = TunnelProviderManager(tunnelConfiguration: tunnelConfiguration)
         for tunnel in tunnels {
             tunnel.index = tunnel.index + 1