error when no upstream set in `contract.dhall` and running `themis-contract upstream diff`

[zramsay]

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x13c826f]

goroutine 1 [running]:
github.com/informalsystems/themis-contract/pkg/themis-contract.(*Contract).UpstreamDiff(0xc000092000, 0x14dfd8a, 0x4, 0xc0000744b0, 0x0, 0x0, 0x0)
	/private/tmp/themis-contract-20201210-7354-9tsu4c/themis-contract-0.2.4/pkg/themis-contract/contract.go:564 +0x6f
main.upstreamDiffCmd.func1(0xc000160dc0, 0x180f0d8, 0x0, 0x0)
	/private/tmp/themis-contract-20201210-7354-9tsu4c/themis-contract-0.2.4/cmd/themis-contract/upstream.go:41 +0xa5
github.com/spf13/cobra.(*Command).execute(0xc000160dc0, 0x180f0d8, 0x0, 0x0, 0xc000160dc0, 0x180f0d8)
	/Users/stars/Library/Caches/Homebrew/go_mod_cache/pkg/mod/github.com/spf13/cobra@v1.0.0/command.go:846 +0x2c2
github.com/spf13/cobra.(*Command).ExecuteC(0xc000132840, 0x0, 0x0, 0xc00014df48)
	/Users/stars/Library/Caches/Homebrew/go_mod_cache/pkg/mod/github.com/spf13/cobra@v1.0.0/command.go:950 +0x375
github.com/spf13/cobra.(*Command).Execute(...)
	/Users/stars/Library/Caches/Homebrew/go_mod_cache/pkg/mod/github.com/spf13/cobra@v1.0.0/command.go:887
main.main()
	/private/tmp/themis-contract-20201210-7354-9tsu4c/themis-contract-0.2.4/cmd/themis-contract/main.go:14 +0xc5