yaml-language-server completion & validation not working

## Information

**VIM version**

```text
:version
VIM - Vi IMproved 9.0 (2022 Jun 28, compiled Feb 7 2023 12:17:42)
macOS version - arm64
```
Operating System: macOS 13.4.1

## What went wrong

1. Completion is not working.
Create an empty `.github/workflows/test.yml` file and invoke omnicomplete ``.
Expected relevant suggestions, like `on` and `jobs`, but nothing happens.
`:ALEInfo` shows LSP error `(Errors for yaml-language-server)
Request textDocument/completion failed with message: Cannot read properties of undefined (reading 'length')`

2. Validation against JSON schema is not working.
Create the following `.github/workflows/test.yml` file:

```yaml
---
on
jobs
```

Expected ALE to show errors on lines 2 & 3, but nothing happens.

## Debugging Attempts

1. I tried to rule out user (mis)configuration issues by defining

```viml
let b:ale_yaml_ls_config = {'yaml': {'schemaStore': { 'enable': v:true, 'url': 'https://www.schemastore.org/json/' }, 'completion': v:true, 'format': { 'enable': v:true }, 'validate': v:true } }
```

2. I tried to rule out yaml LSP issues by testing a different [lsp](https://github.com/yegappan/lsp) client. yaml-language-server provided completion and validation without issues using `yegappan/lsp` client with this LSP configuration:

```viml
call LspAddServer([#{
\ name: 'yaml-language-server',
\ filetype: ['yaml'],
\ path: '/opt/homebrew/bin/yaml-language-server',
\ args: ['--stdio'],
\ }])
```

## Reproducing the bug

1. Create `.github/workflows/test.yaml` file
2. Type invalid yaml syntax, like:

```yaml
---
f
o
o
```

or invalid GitHub Workflow definition, like:

```yaml
---
on
jobs
```

No completion suggestions provided and no errors shown.

### :ALEInfo

Expand

Current Filetype: yaml
Available Linters: ['actionlint', 'circleci', 'gitlablint', 'yaml-language-server', 'spectral', 'swaglint', 'yamllint']
Enabled Linters: ['yaml-language-server', 'spectral', 'yamllint']
Ignored Linters: []
Suggested Fixers:
'prettier' - Apply prettier to a file.
'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.
'yamlfix' - Fix yaml files with yamlfix.
Linter Variables:

let g:ale_yaml_ls_config = {}
let b:ale_yaml_ls_config = {'yaml': {'validate': v:true, 'format': {'enable': v:true}, 'completion': v:true, 'schemaStore': {'url': 'https://www.schemastore.org/json/', 'enable': v:true}}}
let g:ale_yaml_ls_executable = 'yaml-language-server'
let g:ale_yaml_ls_use_global = 0
let g:ale_yaml_spectral_executable = 'spectral'
let g:ale_yaml_spectral_use_global = 0
let g:ale_yaml_yamllint_executable = 'yamllint'
let g:ale_yaml_yamllint_options = ''
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 = 100
let g:ale_completion_enabled = 0
let g:ale_completion_max_suggestions = 50
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 = 1
let g:ale_fixers = {'*': ['remove_trailing_lines', 'trim_whitespace']}
let b:ale_fixers = ['prettier']
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_neovim_diagnostics_api = 0
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
LSP Error Messages:

(Errors for yaml-language-server)
Request textDocument/completion failed with message: Cannot read properties of undefined (reading 'length')
Request textDocument/completion failed with message: Cannot read properties of undefined (reading 'length')
Request textDocument/completion failed with message: Cannot read properties of undefined (reading 'length')
Request textDocument/completion failed with message: Cannot read properties of undefined (reading 'length')
Command History:

(executable check - success) yaml-language-server
(executable check - failure) spectral
(executable check - failure) yamllint
(started) ['/opt/homebrew/bin/bash', '-c', '''yaml-language-server'' --stdio']