test/cli: Use empty array as empty output of images/json

[vvoland]

Tests mocking the output of GET images/json with fakeClient used an array with one empty element as an empty response. Change it to just an empty array.

See comment: #4046 (comment)

Looks like all of these were added as part of the same commit (e779309), that commit was migrated from the moby/moby repository; original one was moby/moby@b2551c6 (moby/moby#32248)

The only possible reasoning I could think of was if it was some attempt to match that the API returns always returns a slice (even if there's no images); the response of the API (tested on docker 17.06 to be sure) is;

curl --unix-socket /var/run/docker.sock 'http://foo/images/json'
[]

But that also doesn't match, because it doesn't return (e.g.) [{}] (single entry).

Other explanation could be to work with the assertion library used at the time; there's a mention that we switched libraries for that at the time; moby/moby#32248 (comment) so perhaps the old one had some issue?

by @thaJeztah

[codecov-commenter]

Codecov Report

Merging #4050 (a1953e1) into master (dfb36ea) will increase coverage by 0.01%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4050      +/-   ##
==========================================
+ Coverage   59.16%   59.17%   +0.01%     
==========================================
  Files         287      287              
  Lines       24724    24725       +1     
==========================================
+ Hits        14627    14631       +4     
+ Misses       9212     9210       -2     
+ Partials      885      884       -1     

[thaJeztah]

LGTM, thanks!