BUG #13884: array_to_json() works incorrectly for non-0-based arrays

The following bug has been logged on the website:

Bug reference: 13884
Logged by: xtracoder
Email address: xtracoder(at)gmail(dot)com
PostgreSQL version: 9.5.0
Operating system: Windows
Description:

When postgres array has no elements at the beginning, array_to_json()
generates json array starting at the first initialized value

Steps to reproduce
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DO $$
DECLARE
arr real[];
jsarr jsonb;
arr2 real[];
BEGIN
arr[3] = 1.1;
arr[4] = 1.1;
raise notice 'arr = %', arr;

jsarr = array_to_json(arr);
raise notice 'arr = %', jsarr;

arr[0] = 1.1;
raise notice 'arr = %', arr;

jsarr = array_to_json(arr);
raise notice 'arr = %', jsarr;
END $$

Actual result
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1. NOTICE: arr = [3:4]={1.1,1.1}
2. NOTICE: arr = [1.1, 1.1]
3. NOTICE: arr = [0:4]={1.1,NULL,NULL,1.1,1.1}
4. NOTICE: arr = [1.1, null, null, 1.1, 1.1]

Expected result
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2. NOTICE: arr = [null, null, null, 1.1, 1.1]