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 gatsbyjs/gatsby#25734

yarn.lock

@@ -1729,12 +1729,7 @@
   dependencies:
     "@types/node" "*"
 
-"@types/node@*":
-  version "14.0.14"
-  resolved "https://registry.yarnpkg.com/@types/node/-/node-14.0.14.tgz#24a0b5959f16ac141aeb0c5b3cd7a15b7c64cbce"
-  integrity sha512-syUgf67ZQpaJj01/tRTknkMNoBBLWJOBODF0Zm4NrXmiSuxjymFrxnTu1QVYRubhVkRcZLYZG8STTwJRdVm/WQ==
-
-"@types/node@^8.5.7":
+"@types/node@*", "@types/node@^8.5.7":
   version "8.10.61"
   resolved "https://registry.yarnpkg.com/@types/node/-/node-8.10.61.tgz#d299136ce54bcaf1abaa4a487f9e4bedf6b0d393"
   integrity sha512-l+zSbvT8TPRaCxL1l9cwHCb0tSqGAGcjPJFItGGYat5oCTiq1uQQKYg5m7AF1mgnEBzFXGLJ2LRmNjtreRX76Q==