fix(lsp): handle non-existent configs in lsp.config/enable

test/functional/plugin/lsp_spec.lua

@@ -6370,5 +6370,35 @@ describe('LSP', function()
         )
       end)
     end)
+
+    it('validates config on attach', function()
+      local tmp1 = t.tmpname(true)
+      exec_lua(function()
+        vim.lsp.log._set_filename(fake_lsp_logfile)
+      end)
+
+      local function test_cfg(cfg, err)
+        exec_lua(function()
+          vim.lsp.config['foo'] = {}
+          vim.lsp.config('foo', cfg)
+          vim.lsp.enable('foo')
+          vim.cmd.edit(assert(tmp1))
+          vim.bo.filetype = 'foo'
+        end)
+
+        retry(nil, 1000, function()
+          assert_log(err, fake_lsp_logfile)
+        end)
+      end
+
+      test_cfg({
+        cmd = { 'lolling' },
+      }, 'cannot start foo due to config error: .* lolling is not executable')
+
+      test_cfg({
+        cmd = { 'cat' },
+        filetypes = true,
+      }, 'cannot start foo due to config error: .* filetypes: expected table, got boolean')
+    end)
   end)
 end)