BUG #14826: Malformed array dimensions for empty arrays in return column "lexemes" of ts_debug()

The following bug has been logged on the website:

Bug reference: 14826
Logged by: Erwin Brandstetter
Email address: brsaweda(at)gmail(dot)com
PostgreSQL version: 9.6.5
Operating system: any
Description:

Empty strings in this column seem to be "out of this world" - comparison to
'{}'::text[] returns FALSE:

test=# SELECT token, lexemes, lexemes = '{}'::text[]
test-# FROM ts_debug('a title');
token | lexemes | ?column?
-------+---------+----------
a | {} | f
| |
title | {titl} | f

The root of the problem seems to be malformed array dimensions.
array_dims(lexemes) returns [1:0], while I would expect NULL:

test=# SELECT token, dictionaries, lexemes
test-# , array_dims(lexemes) AS lex_dims
test-# , array_dims(dictionaries) AS dict_dims
test-# FROM ts_debug('a title');
token | dictionaries | lexemes | lex_dims | dict_dims
-------+----------------+---------+----------+-----------
a | {english_stem} | {} | [1:0] | [1:1]
| {} | | |
title | {english_stem} | {titl} | [1:1] | [1:1]

I tested in versions 8.4 to 10 Beta, always the same result.

Related question on dba.stackexchange:
https://dba.stackexchange.com/questions/186715/how-do-i-select-arrays-that-are-not-empty

Regards
Erwin Brandstetter