Provider crashes when script returns null as value in JSON state object

[pacon-vib]

Thankyou for this great provider!

When I'm running some command-line tools that return JSON objects, but some fields are sometimes filled with null, the null causes the shell provider to panic.

Here's a minimal Terraform config that will cause a panic:

resource "shell_script" "demo" {
  lifecycle_commands {
    create = "echo '{\"baz\": null}' >&3"
    delete = "true"
  }
}

Here's an extract from crash.log; I can submit more if need be:

2020-02-18T10:23:58.730+1100 [DEBUG] plugin.terraform-provider-shell_v0.1.3: panic: interface conversion: interface {} is nil, not string
2020-02-18T10:23:58.730+1100 [DEBUG] plugin.terraform-provider-shell_v0.1.3: 
2020-02-18T10:23:58.730+1100 [DEBUG] plugin.terraform-provider-shell_v0.1.3: goroutine 131 [running]:
2020-02-18T10:23:58.730+1100 [DEBUG] plugin.terraform-provider-shell_v0.1.3: github.com/scottwinkler/terraform-provider-shell/shell.parseJSON(0xc000290000, 0x2000, 0x2000, 0x1, 0x1, 0x2000)
2020-02-18T10:23:58.730+1100 [DEBUG] plugin.terraform-provider-shell_v0.1.3:    /Users/swinkler/go/src/github.com/scottwinkler/terraform-provider-shell/shell/utility.go:53 +0x2b7

Shell scripts are only supposed to return maps of strings to strings in the state, so returning null is naughty, but I would like to propose changing the behaviour to turn those nulls into empty strings, instead of panicking.

For now I'm raising an issue but my colleague is working on a pull request as well.