fix(lsp): warn on missing config in :checkhealth #33087

Problem
When calling `:checkhealth vim.lsp` after the user has enabled a language
server with `vim.lsp.enable` that has no configuration a runtime error
is hit because the code expects for a configuration to exist.

Solution:
Check if a configuration was returned before parsing it, if it isn't
returned then warn the user that the server has been enabled but a
configuration was not found.

(cherry picked from commit 5554fcc286)

runtime/lua/vim/lsp/health.lua

@@ -187,6 +187,11 @@ local function check_enabled_configs()
     local config = vim.lsp.config[name]
     local text = {} --- @type string[]
     text[#text + 1] = ('%s:'):format(name)
+    if not config then
+      report_warn(
+        ("'%s' config not found. Ensure that vim.lsp.config('%s') was called."):format(name, name)
+      )
+    else
       for k, v in
         vim.spairs(config --[[@as table<string,any>]])
       do
@@ -209,6 +214,7 @@ local function check_enabled_configs()
           text[#text + 1] = ('- %s: %s'):format(k, v_str)
         end
       end
+    end
     text[#text + 1] = ''
     report_info(table.concat(text, '\n'))
   end