WaitingRoomSettings - fix interface {} is nil, not string error

[tamas-jozsa]

running $ ./cf-terraforming generate --resource-type "cloudflare_waiting_room_settings"

results in the following error

panic: interface conversion: interface {} is nil, not string

goroutine 1 [running]:
github.com/cloudflare/cf-terraforming/internal/app/cf-terraforming/cmd.init.generateResources.func2(0x1400024ca00?, {0x1009f18b8?, 0x4?, 0x1009f1840?})
        /Users/tjozsa/cf-repos/github/cf-terraforming/internal/app/cf-terraforming/cmd/generate.go:1198 +0x4cb0
github.com/spf13/cobra.(*Command).execute(0x101040200, {0x1400007bfa0, 0x2, 0x2})
        pkg/mod/github.com/spf13/[email protected]/command.go:987 +0x814
github.com/spf13/cobra.(*Command).ExecuteC(0x10103ff20)
        pkg/mod/github.com/spf13/[email protected]/command.go:1115 +0x344
github.com/spf13/cobra.(*Command).Execute(...)
        pkg/mod/github.com/spf13/[email protected]/command.go:1039
github.com/cloudflare/cf-terraforming/internal/app/cf-terraforming/cmd.Execute()
        /Users/tjozsa/cf-repos/github/cf-terraforming/internal/app/cf-terraforming/cmd/root.go:30 +0x24
main.main()
        /Users/tjozsa/cf-repos/github/cf-terraforming/cmd/cf-terraforming/main.go:8 +0x1c

This PR aims to fix that.