Goss crashes when json or junit output is used with io.Readers

Describe the bug When using advanced matching using io.Readers, goss validate panics when the json or junit output is used.

How To Reproduce

# file goss.yaml
command:
  testing:
    exec: printf "oneline\nsecondline\nthirdline"
    exit-status: 0
    stdout:
      and:
        - contain-element: "oneline"
        - contain-element: "thirdline

Then run

$ for type in documentation json junit nagios prometheus rspecish silent structured tap; do echo ${type}; goss validate -f ${type} >/dev/null;  done

to validate the above tests with each available output format.

Expected Behavior No crash on these two output formats.

Actual Behavior Test crashes on the json and junit outputs:

root@moep:~# for type in documentation json junit nagios prometheus rspecish silent structured tap; do echo ${type}; goss validate -f ${type} >/dev/null;  done
documentation
json
panic: interface conversion: interface {} is nil, not float64

goroutine 1 [running]:
github.com/goss-org/goss/outputs.Json.Output({}, {0xc86920, 0xc00012e008}, 0xc0002e8240?, {{0x11ba608?, 0xc000289880?, 0xc0002b3280?}})
        /home/travis/gopath/src/github.com/goss-org/goss/outputs/json.go:62 +0x104a
github.com/goss-org/goss.ValidateConfig(0xc00031e200, 0xc0002b2f00)
        /home/travis/gopath/src/github.com/goss-org/goss/validate.go:142 +0x42a
github.com/goss-org/goss.Validate(0xc00031e200)
        /home/travis/gopath/src/github.com/goss-org/goss/validate.go:111 +0x85
main.main.func1(0xc00029e6e0?)
        /home/travis/gopath/src/github.com/goss-org/goss/cmd/goss/goss.go:151 +0x25
github.com/urfave/cli.HandleAction({0xaaa1a0?, 0xbddc50?}, 0x8?)
        /home/travis/gopath/pkg/mod/github.com/urfave/cli@v1.22.14/app.go:524 +0x50
github.com/urfave/cli.Command.Run({{0xb8f1c5, 0x8}, {0x0, 0x0}, {0xc000376360, 0x1, 0x1}, {0xb95bee, 0xf}, {0x0, ...}, ...}, ...)
        /home/travis/gopath/pkg/mod/github.com/urfave/cli@v1.22.14/command.go:175 +0x67b
github.com/urfave/cli.(*App).Run(0xc0002be1c0, {0xc000132040, 0x4, 0x4})
        /home/travis/gopath/pkg/mod/github.com/urfave/cli@v1.22.14/app.go:277 +0xb87
main.main()
        /home/travis/gopath/src/github.com/goss-org/goss/cmd/goss/goss.go:408 +0x2078
junit
panic: interface conversion: interface {} is nil, not float64

goroutine 1 [running]:
github.com/goss-org/goss/outputs.JUnit.Output({}, {0xc86920, 0xc000012018}, 0xc00028e240?, {{0x11ba608?, 0xc0000c98d0?, 0xc0000cb300?}})
        /home/travis/gopath/src/github.com/goss-org/goss/outputs/junit.go:51 +0xe8c
github.com/goss-org/goss.ValidateConfig(0xc0002c2200, 0xc0000caf80)
        /home/travis/gopath/src/github.com/goss-org/goss/validate.go:142 +0x42a
github.com/goss-org/goss.Validate(0xc0002c2200)
        /home/travis/gopath/src/github.com/goss-org/goss/validate.go:111 +0x85
main.main.func1(0xc0000d26e0?)
        /home/travis/gopath/src/github.com/goss-org/goss/cmd/goss/goss.go:151 +0x25
github.com/urfave/cli.HandleAction({0xaaa1a0?, 0xbddc50?}, 0x8?)
        /home/travis/gopath/pkg/mod/github.com/urfave/cli@v1.22.14/app.go:524 +0x50
github.com/urfave/cli.Command.Run({{0xb8f1c5, 0x8}, {0x0, 0x0}, {0xc0002f8630, 0x1, 0x1}, {0xb95bee, 0xf}, {0x0, ...}, ...}, ...)
        /home/travis/gopath/pkg/mod/github.com/urfave/cli@v1.22.14/command.go:175 +0x67b
github.com/urfave/cli.(*App).Run(0xc0000e21c0, {0xc0000340c0, 0x4, 0x4})
        /home/travis/gopath/pkg/mod/github.com/urfave/cli@v1.22.14/app.go:277 +0xb87
main.main()
        /home/travis/gopath/src/github.com/goss-org/goss/cmd/goss/goss.go:408 +0x2078
nagios
prometheus
rspecish
silent
structured
tap

I have not tested other matchers/negations or regex but I expect the would also crash.

Environment:

Version of goss: v0.4.2

goss-org / goss

Goss crashes when json or junit output is used with io.Readers #848