fix: load plugin object style without options object (#9647)

<!--
  Q. Which branch should I use for my pull request?
  A. Use `master` branch (probably).

  Q. Which branch if my change is a bug fix for Gatsby v1?
  A. In this case, you should use the `v1` branch

  Q. Which branch if I'm still not sure?
  A. Use `master` branch. Ask in the PR if you're not sure and a Gatsby maintainer will be happy to help :)

  Note: We will only accept bug fixes for Gatsby v1. New features should be added to Gatsby v2.

  Learn more about contributing: https://www.gatsbyjs.org/docs/how-to-contribute/
-->

Fixes issue at the end of #9559, e.g.:
```javascript
module.exports = {
    plugins: [
        {
            resolve: 'gatsby-plugin-react-helmet'
        }
    ]
}
```
where the plugin is loaded using object syntax but no `options` object is defined.

This wasn't caught by the unit test because the issue was occurring after the early out when it's the `___TEST___` plugin.

packages/gatsby/src/bootstrap/load-plugins/__tests__/__snapshots__/load-plugins.js.snap

@@ -87,7 +87,7 @@ Array [
   },
   Object {
     "browserAPIs": Array [],
-    "id": "d5fb39bc-5b13-5925-86de-f0e18549d272",
+    "id": "cc2293db-7635-5675-bb5e-05c41ff28688",
     "name": "gatsby-plugin-page-creator",
     "nodeAPIs": Array [
       "createPagesStatefully",
@@ -179,6 +179,7 @@ Array [
   },
   Object {
     "browserAPIs": Array [],
+    "id": "c731052f-1e62-5905-9fb3-edc9057be8f1",
     "name": "TEST",
     "nodeAPIs": Array [],
     "pluginOptions": Object {
@@ -202,7 +203,7 @@ Array [
   },
   Object {
     "browserAPIs": Array [],
-    "id": "d5fb39bc-5b13-5925-86de-f0e18549d272",
+    "id": "cc2293db-7635-5675-bb5e-05c41ff28688",
     "name": "gatsby-plugin-page-creator",
     "nodeAPIs": Array [
       "createPagesStatefully",

packages/gatsby/src/bootstrap/load-plugins/load.js

@@ -19,6 +19,19 @@ function createFileContentHash(root, globPattern) {
   return hash.digest(`hex`)
 }
 
+/**
+ * Make sure key is unique to plugin options. E.g there could
+ * be multiple source-filesystem plugins, with different names
+ * (docs, blogs).
+ * @param {*} name Name of the plugin
+ * @param {*} pluginObject
+ */
+const createPluginId = (name, pluginObject = null) =>
+  createNodeId(
+    name + (pluginObject ? JSON.stringify(pluginObject.options) : ``),
+    `Plugin`
+  )
+
 /**
  * @typedef {Object} PluginInfo
  * @property {string} resolve The absolute path to the plugin
@@ -49,7 +62,7 @@ function resolvePlugin(pluginName) {
         return {
           resolve: resolvedPath,
           name,
-          id: createNodeId(name, `Plugin`),
+          id: createPluginId(name),
           version:
             packageJSON.version || createFileContentHash(resolvedPath, `**`),
         }
@@ -73,7 +86,7 @@ function resolvePlugin(pluginName) {
 
     return {
       resolve: resolvedPath,
-      id: createNodeId(packageJSON.name, `Plugin`),
+      id: createPluginId(packageJSON.name),
       name: packageJSON.name,
       version: packageJSON.version,
     }
@@ -102,9 +115,11 @@ module.exports = (config = {}) => {
         },
       }
     } else {
+      plugin.options = plugin.options || {}
+
       // Plugins can have plugins.
       const subplugins = []
-      if (plugin.options && plugin.options.plugins) {
+      if (plugin.options.plugins) {
         plugin.options.plugins.forEach(p => {
           subplugins.push(processPlugin(p))
         })
@@ -115,8 +130,11 @@ module.exports = (config = {}) => {
       // Add some default values for tests as we don't actually
       // want to try to load anything during tests.
       if (plugin.resolve === `___TEST___`) {
+        const name = `TEST`
+
         return {
-          name: `TEST`,
+          id: createPluginId(name, plugin),
+          name,
           pluginOptions: {
             plugins: [],
           },
@@ -127,15 +145,7 @@ module.exports = (config = {}) => {
 
       return {
         ...info,
-        // Make sure key is unique to plugin options. E.g there could
-        // be multiple source-filesystem plugins, with different names
-        // (docs, blogs).
-        id: createNodeId(
-          plugin.options
-            ? plugin.name + JSON.stringify(plugin.options)
-            : plugin.name,
-          `Plugin`
-        ),
+        id: createPluginId(info.name, plugin),
         pluginOptions: _.merge({ plugins: [] }, plugin.options),
       }
     }
@@ -164,7 +174,7 @@ module.exports = (config = {}) => {
   // Add the site's default "plugin" i.e. gatsby-x files in root of site.
   plugins.push({
     resolve: slash(process.cwd()),
-    id: createNodeId(`default-site-plugin`, `Plugin`),
+    id: createPluginId(`default-site-plugin`),
     name: `default-site-plugin`,
     version: createFileContentHash(process.cwd(), `gatsby-*`),
     pluginOptions: {