Undefined "amount" in create virtual channel RPC request causes node to crash

[geoknee]

          This actually causes the nitro node to crash, which is something we should look into separately:

{"time":"2023-09-13T06:31:49.184521+01:00","level":"INFO","msg":"handling new objective request","address":"0xAAA6628Ec44A8a742987EF3A114dDFE2D4F7aDCE","objective-id":"VirtualFund-0x8f3291c17c49760237b5ba51f92d96c3e1c51bc925f3cb7252172ef9f5eee549"}
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x10 pc=0x1046a1c94]

goroutine 158 [running]:
math/big.(*Int).Set(...)
	/opt/homebrew/Cellar/go/1.21.0/libexec/src/math/big/int.go:98
github.com/statechannels/go-nitro/channel/state/outcome.Allocation.Clone({{0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...}, ...})
	/Users/georgeknee/statechannels/go-nitro/channel/state/outcome/allocation.go:37 +0x54
github.com/statechannels/go-nitro/channel/state/outcome.Allocations.Clone(...)
	/Users/georgeknee/statechannels/go-nitro/channel/state/outcome/allocation.go:63
github.com/statechannels/go-nitro/channel/state/outcome.SingleAssetExit.Clone({{0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...}, ...})
	/Users/georgeknee/statechannels/go-nitro/channel/state/outcome/exit.go:41 +0x110
github.com/statechannels/go-nitro/channel/state/outcome.Exit.Clone(...)
	/Users/georgeknee/statechannels/go-nitro/channel/state/outcome/exit.go:75
github.com/statechannels/go-nitro/channel/state.State.Clone({{0x14000153680, 0x3, 0x4}, 0x9e034e285bd8a121, {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...}, ...})
	/Users/georgeknee/statechannels/go-nitro/channel/state/state.go:210 +0x2f4
github.com/statechannels/go-nitro/channel.New({{0x14000153680, 0x3, 0x4}, 0x9e034e285bd8a121, {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...}, ...}, ...)
	/Users/georgeknee/statechannels/go-nitro/channel/channel.go:61 +0x51c
github.com/statechannels/go-nitro/channel.NewVirtualChannel({{0x14000153680, 0x3, 0x4}, 0x9e034e285bd8a121, {0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...}, ...}, ...)
	/Users/georgeknee/statechannels/go-nitro/channel/virtual.go:30 +0x12c
github.com/statechannels/go-nitro/protocols/virtualfund.constructFromState(0x0?, {{0x14000153680, 0x3, 0x4}, 0x9e034e285bd8a121, {0x0, 0x0, 0x0, 0x0, 0x0, ...}, ...}, ...)
	/Users/georgeknee/statechannels/go-nitro/protocols/virtualfund/virtualfund.go:202 +0x134
github.com/statechannels/go-nitro/protocols/virtualfund.NewObjective({{0x1400050e210, 0x1, 0x1}, {0xbb, 0xb6, 0x76, 0xf9, 0xcf, 0xf8, 0xd2, ...}, ...}, ...)
	/Users/georgeknee/statechannels/go-nitro/protocols/virtualfund/virtualfund.go:155 +0x234
github.com/statechannels/go-nitro/node/engine.(*Engine).handleObjectiveRequest(0x1400091a240, {0x105b4fa40?, 0x1400048aa80?})
	/Users/georgeknee/statechannels/go-nitro/node/engine/engine.go:450 +0xf58
github.com/statechannels/go-nitro/node/engine.(*Engine).run(0x1400091a240, {0x105b53f20, 0x1400065a500})
	/Users/georgeknee/statechannels/go-nitro/node/engine/engine.go:185 +0x274
created by github.com/statechannels/go-nitro/node/engine.New in goroutine 1
	/Users/georgeknee/statechannels/go-nitro/node/engine/engine.go:158 +0x490

Originally posted by @geoknee in https://github.com/statechannels/go-nitro/pull/1689#discussion_r1323996328