use empty slice instead of nil slice when no tags exist for /v1/catalog/services

[abursavich]

the docs for /v1/catalog/services show that the result should be like:

{
    "consul": [],
    "redis": [],
    "postgresql": [
        "master",
        "slave"
    ]
}

but it was actually like:

{
    "consul": null,
    "redis": null,
    "postgresql": [
        "master",
        "slave"
    ]
}

however, other endpoints (/v1/catalog/service/<service> and /v1/catalog/node/<node>) are documented to and actually return nulls for empty tags. for consistency, maybe it makes more sense to update the docs for /v1/catalog/services to reflect the current behavior?

[abursavich]

my point about consistency doesn't quite hold up. it depends on how you configure the service. for the other endpoints mentioned, they will show null if the tags were not configured. but they will show an empty array if the tags were explicitly configured as such.

[armon]

Thanks!