dataNode on TreeNode should not be optional

Environment

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Feature request

This line: https://github.com/palantir/blueprint/blob/develop/packages/core/src/components/tree/treeNode.tsx#L83

    /**
     * An optional custom user object to associate with the node.
     * This property can then be used in the `onClick`, `onContextMenu` and `onDoubleClick`
     * event handlers for doing custom logic per node.
     */
    nodeData?: T;

should not be optional. It prevents type checking. If a user wants that to be optional then they can pass in undefined as a union for the generic parameter

Examples

can't do:

const a: ITreeNode<{type: "a"}> = {} as ITreeNode<{type: "a"}>;

if (a.dataNode.type == "a") // can't do this as a.dataNode can be undefined

if user actually wants undefined they can do ITreeNode<{type: "a"} | undefined>