Lexical version: 0.31.2

Steps To Reproduce

1. Use a TreeView from @lexical/react/LexicalTreeView an supply a custom customPrintNode function.
2. (Implicitly) return undefined to get the default behaviour for nodes whose representation you don't want to override.
3. Type error due to CustomPrintNodeFn return type being string.

The current behavior

TypeScript type-checking fails:

error TS2322: Type '(node: LexicalNode) => string | undefined' is not assignable to type 'CustomPrintNodeFn'.
       Type 'string | undefined' is not assignable to type 'string'.
         Type 'undefined' is not assignable to type 'string'.

The expected behavior

Type checking succeeds.

More info

The code allows returning undefined from customPrintNode to get the default behaviour: