Exception in 'rebar3 hex info' when some meta values are 'null'

It frequently happens that some values (contributors, maintainers, licenses, links) in 'meta' are null instead of list. In this case rebar3 hex info <package> return error

$ DEBUG=1 rebar3 hex info poison
===> Load global config file /home/seriy/.config/rebar3/rebar.config
===> Expanded command sequence to be run: []
===> Expanded command sequence to be run: [{hex,info}]
poison
  Releases: 2.1.0, 2.0.1, 2.0.0, 1.5.2, 1.5.1, 1.5.0, 1.4.0, 1.3.1, 1.3.0, 1.2.1, 1.2.0, 1.1.1, 1.1.0, 1.0.3, 1.0.2, 1.0.1, 1.0.0
===> Uncaught error in rebar_core. Run with DEBUG=1 to see stacktrace
===> Uncaught error: function_clause
===> Stack trace to the error location: [{rebar3_hex_info,
                                                 '-join/1-lc$^0/1-0-',
                                                 [nil],
                                                 [{file,
                                                   "/home/***/.cache/rebar3/plugins/rebar3_hex/src/rebar3_hex_info.erl"},
                                                  {line,184}]},
                                                {rebar3_hex_info,join,1,
                                                 [{file,
                                                   "/home/***/.cache/rebar3/plugins/rebar3_hex/src/rebar3_hex_info.erl"},
                                                  {line,184}]},
<...>

$ rebar3 report "rebar3 hex info poison"
Rebar3 report
 version 3.1.0
 generated at 2016-05-04T22:15:08+00:00
=================
Please submit this along with your issue at https://github.com/erlang/rebar3/issues (and feel free to edit out private information, if any)
-----------------
Task: rebar3
Entered as:
  rebar3 hex info poison
-----------------
Operating System: i686-pc-linux-gnu
ERTS: Erlang/OTP 18 [erts-7.3] [source] [smp:4:4] [async-threads:0] [kernel-poll:false]
Root Directory: /usr/lib/erlang
Library directory: /usr/lib/erlang/lib
-----------------
Loaded Applications:
bbmustache: 1.0.4
certifi: 0.4.0
cf: 0.2.1
compiler: 6.0.3
crypto: 3.6.3
cth_readable: 1.2.2
dialyzer: 2.9
edoc: 0.7.18
erlware_commons: 0.19.0
eunit: 2.2.13
eunit_formatters: 0.3.1
getopt: 0.8.2
inets: 6.2
kernel: 4.2
providers: 1.6.0
public_key: 1.1.1
relx: 3.17.0
sasl: 2.7
snmp: 5.2.2
ssl_verify_hostname: 1.0.5
stdlib: 2.8
syntax_tools: 1.7
tools: 2.8.3

-----------------
Escript path: /usr/local/bin/rebar3
Providers:
  app_discovery as clean compile compile config cover ct cut deps dialyzer do docs edoc escriptize eunit help info install install_deps key list lock new owner path pkgs publish release relup report search shell state tar tree unlock update upgrade upgrade upgrade user version xref

erlef / rebar3_hex

Exception in 'rebar3 hex info' when some meta values are 'null' #46