Omni completion does not work without first manually calling ale_complete

## Information

**VIM version**

VIM - Vi IMproved 8.2 (2019 Dec 12, compiled May 2 2021 18:02:30)
MS-Windows 32-bit console version

Operating System: Windows 11

## What went wrong

I have omni completion configured to use ALE as described in the README:

```
set omnifunc=ale#completion#OmniFunc
```

However, triggering omni completion never works. It always says this:

```
-- Omni completion (^O^N^P) Pattern not found
```

I can fix this by first calling `ale_complete`. For example, if I add this line to my `.vimrc`:

```
imap (ale_complete)
```

Then if I press `` first, afterwards omni completion will work as expected.

Auto-completion also works if enabled like this:

```
let g:ale_completion_enabled = 1
```

But this is **not** what I want: I don't want automatic completions to pop up, I only want completions when manually triggered with ``.

## Reproducing the bug

1. Configure omni completion to use ALE as described in the README: `set omnifunc=ale#completion#OmniFunc`
2. Try to trigger omni completion: press ``
3. Omni completion fails with "Pattern not found"

### :ALEInfo

Expand

Current Filetype: c
Available Linters: ['cc', 'ccls', 'clangd', 'clangtidy', 'cppcheck', 'cpplint', 'cquery', 'cspell', 'flawfinder']
Linter Aliases:
'cc' -> ['gcc', 'clang']
Enabled Linters: ['cc', 'ccls', 'clangd', 'clangtidy', 'cppcheck', 'cpplint', 'cquery', 'cspell', 'flawfinder']
Ignored Linters: []
Suggested Fixers:
'astyle' - Fix C/C++ with astyle.
'clang-format' - Fix C, C++, C#, CUDA, Java, JavaScript, JSON, ObjectiveC and Protobuf files with clang-format.
'clangtidy' - Fix C/C++ and ObjectiveC files with clang-tidy.
'remove_trailing_lines' - Remove all blank lines at the end of a file.
'trim_whitespace' - Remove all trailing whitespace characters at the end of every line.
'uncrustify' - Fix C, C++, C#, ObjectiveC, ObjectiveC++, D, Java, Pawn, and VALA files with uncrustify.
Linter Variables:

let g:ale_c_always_make = 0
let g:ale_c_build_dir = ''
let g:ale_c_build_dir_names = ['build', 'bin']
let g:ale_c_cc_executable = ''
let g:ale_c_cc_header_exts = ['h']
let g:ale_c_cc_options = '-std=c11 -Wall'
let g:ale_c_cc_use_header_lang_flag = -1
let g:ale_c_ccls_executable = 'ccls'
let g:ale_c_ccls_init_options = {}
let g:ale_c_clangd_executable = 'clangd'
let g:ale_c_clangd_options = ''
let g:ale_c_clangtidy_checks = []
let g:ale_c_clangtidy_executable = 'clang-tidy'
let g:ale_c_clangtidy_extra_options = ''
let g:ale_c_clangtidy_options = ''
let g:ale_c_cppcheck_executable = 'cppcheck'
let g:ale_c_cppcheck_options = '--enable=style'
let g:ale_c_cpplint_executable = 'cpplint'
let g:ale_c_cpplint_options = ''
let g:ale_c_cquery_cache_directory = 'C:\Users\cac04\.cache\cquery'
let g:ale_c_cquery_executable = 'cquery'
let g:ale_c_flawfinder_error_severity = 6
let g:ale_c_flawfinder_executable = 'flawfinder'
let g:ale_c_flawfinder_minlevel = 1
let g:ale_c_flawfinder_options = ''
let g:ale_c_parse_compile_commands = 1
let g:ale_c_parse_makefile = 0
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_disable_lsp = 0
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 = 1
let g:ale_lint_on_filetype_changed = 1
let g:ale_lint_on_insert_leave = 1
let g:ale_lint_on_save = 1
let g:ale_lint_on_text_changed = 'normal'
let g:ale_linter_aliases = {}
let g:ale_linters = {}
let g:ale_linters_explicit = 0
let g:ale_linters_ignore = {}
let g:ale_list_vertical = 0
let g:ale_list_window_size = 10
let g:ale_loclist_msg_format = '%code: %%s'
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_root = {}
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 - failure) clang
(executable check - failure) gcc
(executable check - failure) ccls
(executable check - success) clangd
(executable check - failure) clang-tidy
(executable check - failure) cppcheck
(executable check - failure) cpplint
(executable check - failure) cquery
(executable check - failure) cspell
(executable check - failure) flawfinder
(started) 'cmd /s/c "clangd"'