fix: undefined check (#962)

| [![PR App][icn]][demo] | RM-10062 |
| :--------------------: | :------: |

## 🧰 Changes

Fixes parsing `undefined` attributes.

## 🧬 QA & Testing

- [Broken on production][prod].
- [Working in this PR app][demo].

[demo]: https://markdown-pr-PR_NUMBER.herokuapp.com
[prod]: https://SUBDOMAIN.readme.io
[icn]:
https://user-images.githubusercontent.com/886627/160426047-1bee9488-305a-4145-bb2b-09d8b757d38a.svg

__tests__/compilers/images.test.ts

@@ -24,4 +24,10 @@ describe('image compiler', () => {
 
     expect(mdx(mdast(doc))).toMatch(doc);
   });
+
+  it('correctly serializes an Image component with an undefined expression attributes back to MDX', () => {
+    const doc = '<Image border={undefined} />';
+
+    expect(mdx(mdast(doc))).toMatch('![]()');
+  });
 });

processor/utils.ts

@@ -66,7 +66,11 @@ export const getHPropKeys = <T>(node: Node): any => {
 export const getAttrs = <T>(jsx: MdxJsxFlowElement | MdxJsxTextElement): any =>
   jsx.attributes.reduce((memo, attr) => {
     if ('name' in attr) {
-      memo[attr.name] = typeof attr.value !== 'string' ? JSON.parse(attr.value.value) : attr.value;
+      if (typeof attr.value === 'string') {
+        memo[attr.name] = attr.value;
+      } else if (attr.value.value !== 'undefined') {
+        memo[attr.name] = JSON.parse(attr.value.value);
+      }
     }
 
     return memo;
@@ -157,6 +161,8 @@ export const toAttributes = (object: Record<string, any>, keys: string[] = []):
 
     if (typeof v === 'string') {
       value = v;
+    } else if (typeof v === 'undefined') {
+      return;
     } else {
       /* values can be null, undefined, string, or a expression, eg:
        *