BUG #16107: string_agg looses first item

The following bug has been logged on the website:

Bug reference:      16107
Logged by:          Andrew Wheelwright
Email address:      andrew.wheelwright@familysearch.org
PostgreSQL version: 11.5
Operating system:   Linux
Description:

I ran into a scenario where I found results getting dropped from string
aggregation.  Here's a basic example which reproduces the problem on three
different servers running PostgreSQL 9.6.6, 10.6, and 11.5, respectively.  I
don't have an instance running version 12.

with dataset as (
    select 'One' "Label", 1 "ID"
    union
    select 'Two' "Label", 2 "ID"
    union
    select 'Three' "Label", 3 "ID"
)
select
    string_agg(', ', "Label" order by "ID") "String Aggregated Labels",
    array_agg("Label" order by "ID") "Array Aggregated Labels"
from
    dataset
;

Which renders the following result:
String Aggregated Labels: `, Two, Three, `
Array Aggregated Labels: `{One,Two,Three}`

The value "One" is missing from the string aggregate result and there is a
dangling separator on both ends of the string.