Fix index error when a new service (or one that doesn't have yesterday's cost)

[rwilson-release]

Before:

{
    "errorMessage": "list index out of range",
    "errorType": "IndexError",
    "stackTrace": [
        "  File \"/var/task/handler.py\", line 101, in report_cost\n    buffer += f\"{service_name:{longest_name_len}} ${costs[-1]:8,.2f} {delta(costs):4.0f}% {sparkline(costs):7}\\n\"\n",
        "  File \"/var/task/handler.py\", line 31, in delta\n    if (costs[-1] >= 1 and costs[-2] >= 1):\n"
    ]
}

After:

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