Add useful error on empty presets

Empty presets are most likely a mistake by the user. At best, they do nothing.
This improves the situation for humans that make mistakes, by throwing a useful error.

Closes GH-200.
Closes GH-202.

Reviewed-by: Christian Murphy <christian.murphy.42@gmail.com>
Reviewed-by: Remco Haszing <remcohaszing@gmail.com>
Reviewed-by: JounQin <admin@1stg.me>

Author: wooorm

lib/index.js

@@ -197,6 +197,12 @@ function base() {
      * @returns {void}
      */
     function addPreset(result) {
+      if (!('plugins' in result) && !('settings' in result)) {
+        throw new Error(
+          'Expected usable value but received an empty preset, which is probably a mistake: presets typically come with `plugins` and sometimes with `settings`, but this has neither'
+        )
+      }
+
       addList(result.plugins)
 
       if (result.settings) {

test/use.js

@@ -291,10 +291,13 @@ test('use(preset)', async (t) => {
     'should throw on invalid `plugins` (2)'
   )
 
-  await t.test('should support empty presets', () => {
-    const processor = unified().use({}).freeze()
-    assert.equal(processor.attachers.length, 0)
-  })
+  assert.throws(
+    () => {
+      unified().use({}).freeze()
+    },
+    /Expected usable value but received an empty preset/,
+    'should throw on empty presets'
+  )
 
   await t.test('should support presets with empty plugins', () => {
     const processor = unified().use({plugins: []}).freeze()