Omitting the parameters/required field from a bundle causes SIGSEGV on installation

[glyn]

According to the CNAB spec, the parameters/required field in a bundle is optional:

required: A list of required parameters. MUST be an array of strings.(OPTIONAL)

The bundle schema confirms this by not declaring the field to be required.

Yet attempting to install a bundle without an explicit empty array for parameters/required results in SIGSEGV e.g.:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x13580aa]

goroutine 1 [running]:
github.com/deislabs/duffle/vendor/github.com/deislabs/cnab-go/bundle.ValuesOrDefaults(0xc00036b410, 0xc0000da540, 0xc0002c7a30, 0x0, 0xc00036b440)
	/Users/gnormington/go/src/github.com/deislabs/duffle/vendor/github.com/deislabs/cnab-go/bundle/bundle.go:139 +0x9a
main.calculateParamValues(0xc0000da540, 0x0, 0x0, 0x34e4b08, 0x0, 0x0, 0x34e4b08, 0x0, 0x0, 0x3, ...)
	/Users/gnormington/go/src/github.com/deislabs/duffle/cmd/duffle/install.go:264 +0x3dd
main.(*installCmd).run(0xc0000da240, 0x1a, 0x24beb40)
	/Users/gnormington/go/src/github.com/deislabs/duffle/cmd/duffle/install.go:139 +0x41e
main.newInstallCmd.func1(0xc0001dcf00, 0xc0000e6e40, 0x2, 0x2, 0x0, 0x0)
	/Users/gnormington/go/src/github.com/deislabs/duffle/cmd/duffle/install.go:85 +0xcf
github.com/deislabs/duffle/vendor/github.com/spf13/cobra.(*Command).execute(0xc0001dcf00, 0xc0000e6e00, 0x2, 0x2, 0xc0001dcf00, 0xc0000e6e00)
	/Users/gnormington/go/src/github.com/deislabs/duffle/vendor/github.com/spf13/cobra/command.go:762 +0x465
github.com/deislabs/duffle/vendor/github.com/spf13/cobra.(*Command).ExecuteC(0xc0003c3b80, 0x0, 0xc0003c3b80, 0xc00009a058)
	/Users/gnormington/go/src/github.com/deislabs/duffle/vendor/github.com/spf13/cobra/command.go:852 +0x2ec
github.com/deislabs/duffle/vendor/github.com/spf13/cobra.(*Command).Execute(...)
	/Users/gnormington/go/src/github.com/deislabs/duffle/vendor/github.com/spf13/cobra/command.go:800
main.main()
	/Users/gnormington/go/src/github.com/deislabs/duffle/cmd/duffle/main.go:32 +0x48

Steps to reproduce

1. Unless schema version missing from examples duffle#795 has been fixed, edit examples/helloworld/duffle.json to ensure it has a schema version, e.g.

   "schemaVersion" : "v1.0.0-WD",
   

2. duffle build ./examples/helloworld
3. docker push the resultant invocation image
4. duffle install helloworld helloworld
5. Observe SIGSEGV

[glyn]

Fixed by #50