fix(gatsby-plugin-mdx): support import shorthand syntax (#26198)

The "shorthand" syntax is an import that only imports a path, without declaring any symbols:

```js
import "./foo.css"
```

This happened in the real world so we have to support it.

This fixes #25734

Author: pvdz

packages/gatsby-plugin-mdx/utils/__tests__/__snapshots__/import-parser.js.snap

@@ -985,6 +985,26 @@ Object {
 `;
 
 exports[`regex import scanner syntactic coverage should parse brute force regular case 62 1`] = `
+Object {
+  "input": "import \\"./empty.css\\"",
+  "result": Object {
+    "bindings": Array [],
+    "segments": Array [],
+  },
+}
+`;
+
+exports[`regex import scanner syntactic coverage should parse brute force regular case 63 1`] = `
+Object {
+  "input": "import \\"./empty.css\\"; // This happens in the real world",
+  "result": Object {
+    "bindings": Array [],
+    "segments": Array [],
+  },
+}
+`;
+
+exports[`regex import scanner syntactic coverage should parse brute force regular case 64 1`] = `
 Object {
   "input": "import multi as dong, {foo} from 'bar'
 import as as x, {from as y} from 'bar'",

packages/gatsby-plugin-mdx/utils/__tests__/import-parser.js

@@ -72,6 +72,8 @@ function getBruteForceCases() {
     import foo from 'bar' // commment containing confusing from "chars"
     import foo from 'bar' // import bad from 'imp'
     import foo from 'bar'//next to it
+    import "./empty.css"
+    import "./empty.css"; // This happens in the real world
   `
     .trim()
     .split(/\n/g)
@@ -94,7 +96,11 @@ describe(`regex import scanner`, () => {
         const output = parseImportBindings(input, true)
         const bindings = output.bindings
 
-        expect(output.bindings.length).not.toBe(0)
+        if (input.includes(`empty`)) {
+          expect(output.bindings.length).toBe(0)
+        } else {
+          expect(output.bindings.length).not.toBe(0)
+        }
         // Note: putting everything in the snapshot makes reviews easier
         expect({ input, result: output }).toMatchSnapshot()
         expect(
@@ -288,5 +294,56 @@ import {A} from "@your/name"
         ]
       `)
     })
+
+    it(`double import with shorthand import`, async () => {
+      // Note: the point of this test is to have two back2back imports clustered
+      //       as one pseudo-node in the ast where the first is the short-hand
+      //       version of `import` that declares no symbols.
+
+      const { content } = grayMatter(`
+---
+title: double test
+---
+
+import "./foo.css"
+import Events from "@components/events/events"
+
+<Events />
+      `)
+
+      const compiler = mdx.createCompiler()
+      const fileOpts = { contents: content }
+      const mdast = await compiler.parse(fileOpts)
+
+      const imports = mdast.children.filter(obj => obj.type === `import`)
+
+      // Assert the md parser outputs same mdast (update test if this changes)
+      expect(
+        imports.map(({ type, value }) => {
+          return { type, value }
+        })
+      ).toMatchInlineSnapshot(`
+        Array [
+          Object {
+            "type": "import",
+            "value": "import \\"./foo.css\\"
+        import Events from \\"@components/events/events\\"",
+          },
+        ]
+      `)
+
+      // Take the imports being parsed and feed them to the import parser
+      expect(parseImportBindings(imports[0].value, true))
+        .toMatchInlineSnapshot(`
+        Object {
+          "bindings": Array [
+            "Events",
+          ],
+          "segments": Array [
+            "Events",
+          ],
+        }
+      `)
+    })
   })
 })

packages/gatsby-plugin-mdx/utils/import-parser.js

@@ -7,7 +7,8 @@
  * `from` tail. What's left ought to be a string in the form of
  * `id[ as id] [, id[ as id]]`
  * (where the brackets represent optional repeating parts). The left-most id
- * might also contain star (namespaced import).
+ * might also contain star (namespaced import). This step also strips
+ * `import "foo"` shorthand imports.
  * The second part will trim and split the string on comma, then each segment
  * is split on `as` (in a proper way) and the right-most identifier is returned.
  *
@@ -19,7 +20,7 @@
  */
 function parseImportBindings(importCode, returnSegments = false) {
   const str = importCode.replace(
-    /^\s*import|[{},]|\s*from\s*['"][^'"]*?['"].*?$/gm,
+    /^\s*import\s*['"][^'"]*?['"].*?$|^\s*import|[{},]|\s*from\s*['"][^'"]*?['"].*?$/gm,
     ` , `
   )
   const segments = str