Panic decoding JSON reports with review command

[gsantos-hc]

I'm attempting to review a benchmark report that I earlier saved as a JSON file, but vault-benchmark panics when deserializing the file:

$ vault-benchmark -version
vault-benchmark v0.3.0

$ vault-benchmark review -results_file benchmark-10nvs.json
error reading report: json: cannot unmarshal string into Go value of type vegeta.Metrics
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x8 pc=0x104e83754]

goroutine 1 [running]:
github.com/hashicorp/vault-benchmark/benchmarktests.(*Reporter).ReportTerse(0x0, {0x10534e580, 0x140001a2008})
        github.com/hashicorp/vault-benchmark/benchmarktests/reports.go:117 +0x64
github.com/hashicorp/vault-benchmark/command.(*ReviewCommand).Run(0x14000300540, {0x140001a6060, 0x2, 0x2})
        github.com/hashicorp/vault-benchmark/command/review.go:109 +0x270
github.com/mitchellh/cli.(*CLI).Run(0x140000e0140)
        github.com/mitchellh/[email protected]/cli.go:262 +0x4a8
github.com/hashicorp/vault-benchmark/command.RunCustom({0x140001a6050?, 0x10468dd1c?, 0x1058e86f8?}, 0x0?)
        github.com/hashicorp/vault-benchmark/command/main.go:138 +0x730
github.com/hashicorp/vault-benchmark/command.Run(...)
        github.com/hashicorp/vault-benchmark/command/main.go:41
main.main()
        github.com/hashicorp/vault-benchmark/main.go:20 +0x54

The JSON report was created through something like this:

vault-benchmark run -config benchmark.hcl -report_mode json > benchmark.json