[bug]: Add a nil check before closing the channel in HashMailCourier

[ziggie1984]

When forcefully shutting down and the daemon is in the process of opening a channel I get a panic:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x28 pc=0x1042ba020]

goroutine 2484 [running]:
github.com/lightninglabs/taproot-assets/proof.(*HashMailCourier).Close(0x140002f41e0?)
    github.com/lightninglabs/[email protected]/proof/courier.go:1029 +0x20
github.com/lightninglabs/taproot-assets/tapfreighter.(*ChainPorter).transferReceiverProof.func1({_, _}, {{{{0x60, 0xca, 0x45, 0x2c, 0x1d, 0x1e, 0x5c, 0xab, ...}, ...}, ...}, ...})
    github.com/lightninglabs/[email protected]/tapfreighter/chain_porter.go:854 +0x680
github.com/lightninglabs/taproot-assets/fn.ParSliceErrCollect[...].func1()
    github.com/lightninglabs/[email protected]/fn/concurrency.go:54 +0xa4
golang.org/x/sync/errgroup.(*Group).Go.func1()
    golang.org/x/[email protected]/errgroup/errgroup.go:78 +0x54
created by golang.org/x/sync/errgroup.(*Group).Go in goroutine 2482
    golang.org/x/[email protected]/errgroup/errgroup.go:75 +0x98