fix: guard against markDefs being null

[hjalm8]

Default values during a destructuring assignment are not used when the value is null.

Example from MDN:

const [a = 1] = []; // a is 1
const { b = 2 } = { b: undefined }; // b is 2
const { c = 2 } = { c: null }; // c is null

I encountered this PortableText block:

[
  {
    "_type": "block",
    "style": "normal",
    "_key": "3f2160c15ba9",
    "children": [
      {
        "_type": "span",
        "marks": [
          "em"
        ],
        "text": "Example",
        "_key": "d8bf318b9723"
      }
    ]
  }
]

This caused rendering to crash when using the PortableText component from portabletext/react.
Specifically ./src/buildMarksTree.ts:93 Error: Cannot read properties of null (reading 'find')

[christianhg]

Thank you!