Linter warning on empty line only gets highlighted when signs are turned on

## Information

**VIM version**

VIM - Vi IMproved 8.1 (2018 May 18, compiled Oct 28 2019 12:23:44)
Included patches: 1-2231

Operating System: Ubuntu 18.04.3

## What went wrong

I have a linter warning on an empty line (caused by a "no trailing blank lines" type of rule).

With this config:
```
let g:ale_set_signs = 1
let g:ale_set_highlights = 1
hi link aleerrorline spellbad
hi link alewarningline spellcap
```
ALE correctly sets the sign and highlights the whole empty line.

If I disable signs via `let g:ale_set_signs = 0`, it not only stops setting the sign column (expected behavior), but also stops highlighting linter warnings on empty lines (unexpected).

## Reproducing the bug

Using the config above, open any Ruby file containing a trailing blank line and run the Rubocop linter. This is a specific example but any other linter / file combo that causes a linting warning on an empty line should show the same issue.

### :ALEInfo

Current Filetype: ruby
Available Linters: ['brakeman', 'debride', 'rails_best_practices', 'reek', 'rubocop', 'ruby', 'solargraph', 'sorbet', 'standardrb']
Linter Aliases:
'sorbet' -> ['srb']
Enabled Linters: ['rubocop', 'ruby']
Suggested Fixers:
'remove_trailing_lines' - Remove all blank lines at the end of a file.
'rubocop' - Fix ruby files with rubocop --auto-correct.
'rufo' - Fix ruby files with rufo
'sorbet' - Fix ruby files with srb tc --autocorrect.
'standardrb' - Fix ruby files with standardrb --fix
'trim_whitespace' - Remove all trailing whitespace characters at the end of every line.
Linter Variables:

let g:ale_ruby_rails_best_practices_executable = 'rails_best_practices'
let g:ale_ruby_rails_best_practices_options = ''
let g:ale_ruby_rubocop_executable = 'rubocop'
let g:ale_ruby_rubocop_options = ''
let g:ale_ruby_ruby_executable = 'ruby'
Global Variables:

let g:ale_cache_executable_check_failures = v:null
let g:ale_change_sign_column_color = 0
let g:ale_command_wrapper = ''
let g:ale_completion_delay = v:null
let g:ale_completion_enabled = 0
let g:ale_completion_max_suggestions = v:null
let g:ale_echo_cursor = 1
let g:ale_echo_msg_error_str = 'Error'
let g:ale_echo_msg_format = '%code: %%s'
let g:ale_echo_msg_info_str = 'Info'
let g:ale_echo_msg_warning_str = 'Warning'
let g:ale_enabled = 1
let g:ale_fix_on_save = 0
let g:ale_fixers = {}
let g:ale_history_enabled = 1
let g:ale_history_log_output = 1
let g:ale_keep_list_window_open = 0
let g:ale_lint_delay = 200
let g:ale_lint_on_enter = 0
let g:ale_lint_on_filetype_changed = 1
let g:ale_lint_on_insert_leave = 0
let g:ale_lint_on_save = 1
let g:ale_lint_on_text_changed = 0
let g:ale_linter_aliases = {}
let g:ale_linters = {'ruby': ['ruby', 'rubocop'], 'javascript': ['eslint'], 'eruby': []}
let g:ale_linters_explicit = 0
let g:ale_list_vertical = 0
let g:ale_list_window_size = 10
let g:ale_loclist_msg_format = '%code: %%s'
let g:ale_lsp_root = {}
let g:ale_max_buffer_history_size = 20
let g:ale_max_signs = -1
let g:ale_maximum_file_size = v:null
let g:ale_open_list = 0
let g:ale_pattern_options = v:null
let g:ale_pattern_options_enabled = v:null
let g:ale_set_balloons = 0
let g:ale_set_highlights = 1
let g:ale_set_loclist = 1
let g:ale_set_quickfix = 0
let g:ale_set_signs = 1
let g:ale_sign_column_always = 0
let g:ale_sign_error = '>>'
let g:ale_sign_info = '--'
let g:ale_sign_offset = 1000000
let g:ale_sign_style_error = '>>'
let g:ale_sign_style_warning = '--'
let g:ale_sign_warning = '--'
let g:ale_sign_highlight_linenrs = 0
let g:ale_statusline_format = v:null
let g:ale_type_map = {}
let g:ale_use_global_executables = v:null
let g:ale_virtualtext_cursor = 0
let g:ale_warn_about_trailing_blank_lines = 1
let g:ale_warn_about_trailing_whitespace = 1
Command History:

(executable check - success) rubocop
(finished - exit code 1) ['/bin/bash', '-c', '''rubocop'' --format json --force-exclusion --stdin ''/home/ranko/projects/client-portal/Gemfile'' < ''/tmp/vPxyDJ3/4/Gemfile''']

<<>>
{"metadata":{"rubocop_version":"0.71.0","ruby_engine":"ruby","ruby_version":"2.5.1","ruby_patchlevel":"57","ruby_platform":"x86_64-linux"},"files":[{"path":"Gemfile","offenses":[{"severity":"convention","message":"1 trailing blank lines detected.","cop_name":"Layout/TrailingBlankLines","corrected":false,"location":{"start_line":43,"start_column":1,"last_line":44,"last_column":0,"length":1,"line":43,"column":1}}]}],"summary":{"offense_count":1,"target_file_count":1,"inspected_file_count":1}}
<<>>

(executable check - success) ruby
(finished - exit code 0) ['/bin/bash', '-c', '''ruby'' -w -c -T1 ''/tmp/vPxyDJ3/5/Gemfile''']

<<>>