fix(lsp): support workspace/configuation with no section #27510

The [spec for `workspace/configuration`](https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#workspace_configuration)
marks the `section` property of each item in `items` optional.
Therefore, I believe it violates the spec to skip over items without
a section, because then the length of the results won't match the length
of a valid `items` input. The spec does not elaborate on _what_ to
return in this case, but I don't think it would be controversial to say
that returning the full configuration, as done for an empty string, is
the most natural interpretation.

That empty string case, by the way, was initially [added in
response](5da124fc82)
to a real world implementation requesting it. I don't have a similar
real world implementation to point to for the omitted `section`, but
I would note that `getConfiguration()` from `vscode-languageserver-node`
[defaults to a request with no section](d859bb14d1/server/src/common/configuration.ts (L24-L26))
when called with no arguments. I surmise that this is intended as a way
to retrieve the full configuration.

runtime/lua/vim/lsp/handlers.lua

@@ -230,6 +230,9 @@ RSC['workspace/configuration'] = function(_, params, ctx)
         value = vim.NIL
       end
       table.insert(response, value)
+    else
+      -- If no section is provided, return settings as is
+      table.insert(response, client.settings)
     end
   end
   return response

test/functional/fixtures/fake-lsp-server.lua

@@ -150,9 +150,16 @@ function tests.check_workspace_configuration()
           { section = 'testSetting2' },
           { section = 'test.Setting3' },
           { section = 'test.Setting4' },
+          {},
+          { section = '' },
         },
       })
-      expect_notification('workspace/configuration', { true, false, 'nested', vim.NIL })
+      local all = {
+        testSetting1 = true,
+        testSetting2 = false,
+        test = { Setting3 = 'nested' },
+      }
+      expect_notification('workspace/configuration', { true, false, 'nested', vim.NIL, all, all })
       notify('shutdown')
     end,
   }

test/functional/plugin/lsp_spec.lua

@@ -681,6 +681,8 @@ describe('LSP', function()
               { section = 'testSetting2' },
               { section = 'test.Setting3' },
               { section = 'test.Setting4' },
+              {},
+              { section = '' },
             },
           },
           { method = 'workspace/configuration', client_id = 1 },