empty alternative domain (-alt-domain="") is not possible anymore with 1.7.1

Hello

#### Overview of the Issue

In Consul `1.6.3` we were able to run consul agent with empty alternative domain
either with `-alt-domain=""`
or with `/etc/consul/config.json` that contains
```
{
"alt_domain": "",
...
}
```
It was very handy for developers environments
since they didn't have to remember about current FQDN they work.
like
```
$ dig @localhost -p8600 foo001.node +short
10.4.4.4
```
so having
```
server=/node/127.0.0.1#8600`
server=/service/127.0.0.1#8600
```
in DNSmasq config was very useful so hosts and services were resolved
alternatively for humans like:
```
dig foo001.node
10.4.4.4
dig foo.service
10.4.4.5
```

with `1.7.1` this feature is not possible anymore

#### Reproduction Steps

1. Create a cluster with 1 client node and 1 server node - both with `-alt-domain=""`
2. ask about self hostname like `dig @localhost -p8600 foo001.node +short`

in 1.6.3 works, in 1.7.1 it doesn't work anymore

### Consul info for both Client and Server

Client info

```
agent:
check_monitors = 0
check_ttls = 0
checks = 0
services = 1
build:
prerelease =
revision = 2cf0a3c8
version = 1.7.1
consul:
acl = disabled
known_servers = 1
server = false
runtime:
arch = amd64
cpu_count = 1
goroutines = 48
max_procs = 1
os = linux
version = go1.13.7
serf_lan:
coordinate_resets = 0
encrypted = true
event_queue = 0
event_time = 7
failed = 0
health_score = 0
intent_queue = 0
left = 0
member_time = 164
members = 43
query_queue = 0
query_time = 1
```

### Operating system and Environment details

Ubuntu x86_64

### Log Fragments

```
==> Starting Consul agent...
Version: 'v1.7.1'
Node ID: '851272e1-391c-133c-0499-59d88815c382'
Node name: 'foo001'
Datacenter: 'staging' (Segment: '')
Server: false (Bootstrap: false)
Client Addr: [0.0.0.0] (HTTP: 8500, HTTPS: -1, gRPC: -1, DNS: 8600)
Cluster Addr: 10.4.4.4 (LAN: 8301, WAN: 8302)
Encrypt: Gossip: true, TLS-Outgoing: false, TLS-Incoming: false, Auto-Encrypt-TLS: false
```