Fix underflow when broadcasting storage proof for expired contracts

[n8maninger]

Fixes a panic when reducing the collateral metric of a contract that does not need a storage proof.

panic: underflow

goroutine 39 [running]:
go.sia.tech/core/types.Currency.Sub(...)
      go.sia.tech/[email protected]/types/currency.go:97
, 0x0?}, 0x1, {0xc12ab85287de76a2, 0x29326ed31, 0x1c593c0})
      go.sia.tech/hostd/persist/sqlite/metrics.go:423 +0x479
go.sia.tech/hostd/persist/sqlite.(*Store).ExpireContract.func1({0x15f3c08, 0xc000592460})
      go.sia.tech/hostd/persist/sqlite/contracts.go:458 +0x212
go.sia.tech/hostd/persist/sqlite.(*Store).transaction(0xc000116360, {0x15f35b0, 0xc0008d4690}, 0xc0000f7ba0)
      go.sia.tech/hostd/persist/sqlite/store.go:108 +0x669
go.sia.tech/hostd/persist/sqlite.(*Store).ExpireContract(0x2?, {0xcb, 0x89, 0x9b, 0x90, 0xf2, 0xde, 0xd9, 0x84, 0x88, ...}, ...)
      go.sia.tech/hostd/persist/sqlite/contracts.go:449 +0xbe
go.sia.tech/hostd/host/contracts.(*ContractManager).handleContractAction(0xc0003a8c60, {0xcb, 0x89, 0x9b, 0x90, 0xf2, 0xde, 0xd9, 0x84, 0x88, ...}, ...)
      go.sia.tech/hostd/host/contracts/actions.go:159 +0x195a
go.sia.tech/hostd/persist/sqlite.(*Store).ContractAction(0xc000116360, 0xc0006ba380?, 0xc000592160)
      go.sia.tech/hostd/persist/sqlite/contracts.go:417 +0x423
go.sia.tech/hostd/host/contracts.(*ContractManager).processActions.func1(0xc0003a8c60, 0xc0000f9e80?)
      go.sia.tech/hostd/host/contracts/actions.go:66 +0xaa
go.sia.tech/hostd/host/contracts.(*ContractManager).processActions(0xc0003a8c60)
      go.sia.tech/hostd/host/contracts/actions.go:73 +0xa5
created by go.sia.tech/hostd/host/contracts.NewManager
      go.sia.tech/hostd/host/contracts/manager.go:484 +0x38a