[lexical-devtools-core] Bug Fix: Allow CustomPrintNodeFn to return undefined

[achaljhawar]

Description

• The current behavior is that the CustomPrintNodeFn type is strictly typed to return string. This causes a TypeScript error if your provided function returns undefined to fall back to the default node printing logic, even though the runtime code handles this scenario.
• This PR modifies the CustomPrintNodeFn type definition to be string | undefined. This aligns the type with the existing runtime behavior and resolves the TypeScript error.

Closes #7612

Test plan

Before

If you provided a customPrintNode function like this, you would encounter a TypeScript error:

const myCustomPrintNode: CustomPrintNodeFn = (node) => {
  if (node.getType() === 'custom') {
    return 'Custom node representation';
  }
  return undefined; // TypeScript error: Type 'undefined' is not assignable to type 'string'.
};

After

With the updated type definition, the above code will no longer produce a TypeScript error. The type checking will succeed, reflecting the actual supported behavior of the generateContent function in lexical-devtools-core.

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