how line protocol support inserting empty tag value?

__Proposal:__
I want to migrate data to influxdb from other tsdb. Now I has some data with empty tag value, when wirte it to influxdb and query it, find that channel_name tag value is null. beflow is my test data:
```
{
"measurement": "mysql_slave_status_seconds_behind_master",
"fields": {
"value": 1
},
"tags": {
"channel_name": "",
"key": "1234",
}
}
```
__Current behavior:__
"" tag value inserted to influxdb, value is null

__Desired behavior:__
I desired aflter "" tag value inserted to influxdb, it's value is ""

__Use case:__
test data:
```
{
"measurement": "mysql_slave_status_seconds_behind_master",
"fields": {
"value": 1
},
"tags": {
"channel_name": "",
"key": "1234",
}
}
```
got line ptotocol: mysql_slave_status_seconds_behind_master,key=1234 value=1i

No implementation file or test is identified. Reproduce the supplied empty-tag example using InfluxDB's line protocol, then trace how the empty tag is represented during writing and querying. Done means agreeing on and verifying whether an empty tag value is preserved as "" rather than returned as null.

Written by the indexing model from the issue text.