fix: fix PreProcessor split_by schema

[ZanSara]

Related Issues

• fixes PreProcessor from json config {'split_by': null} gives schema validation problem #3633

Proposed Changes:

• Fix typing of the split_by parameter in all methods

How did you test it?

• Regenerated the schema by hand, which yielded:

"split_by": {
  "title": "Split By",
  "default": "word",
  "enum": [
    "word",
    "sentence",
    "passage"
  ],
  "anyOf": [
    {
      "type": "string"
    },
    {
      "type": "null"
    }
  ]
},

while, for comparison, the current schema looks like:

"split_by": {
  "default": "word",
  "enum": [
    "word",
    "sentence",
    "passage"
  ],
  "title": "Split By",
  "type": "string"
},

Notes for the reviewer

n/a

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added tests that demonstrate the correct behavior of the change
• I've used the conventional commit convention for my PR title
• I documented my code
• I ran pre-commit hooks and fixed any issue

[bogdankostic]

LGTM