fix: guard against nullable markDefs

[bobinska-dev]

Marking markDefs as optional to resolve TS error when markDefs array is missing

[christianhg]

Looks like a safe addition to safe guard against a potential nullable markDef!

[jepezi]

Awesome, I ran into this problem today and about to submit pr but @bobinska-dev beated me to it! In case someone has the same problem, here's context from slack.

Hi! I'm getting an error [ Server ] Error: Cannot read properties of null (reading 'find') when I copy and paste the nested block content in the same field of same document type. The error comes from serializeBlock function in react-portable-text module where it tries to buildMarksTree. I log the data and found that the original field always has "markDefs": [] and "marks": [] set to empty array but the field where I pasted into contain markDefs: null and marks: null. Is this a bug in copy and paste feature or do I have to change any of my schema?

here's the part of the data

"content": [
  {
    "_key": "1d6d03fd419e",
    "_type": "block",
    "style": "normal",
    "markDefs": null, // <----- this is the problem
    "children": [
      {
        "_key": "d0a5a756499b",
        "_type": "span",
        "text": "sample text"
      }
    ],
  },
]