Skip map_expressions rebuild for Extension nodes with empty expressions

[zhuqi-lucas]

Is your feature request related to a problem or challenge?

When an Extension node (e.g. view matching OneOf) has many children but no expressions, LogicalPlan::map_expressions still clones all inputs and calls with_exprs_and_inputs to reconstruct the node — wasted work since there are no expressions to transform.

In our production system with view matching (5 MV candidates × ~15 optimizer rules), this unnecessary rebuild is the dominant cost of the optimizer pass for plans containing Extension nodes.

Describe the solution you'd like

Add an early return in map_expressions when node.expressions() is empty:

LogicalPlan::Extension(Extension { node }) => {
    let raw_exprs = node.expressions();
    if raw_exprs.is_empty() {
        Transformed::no(LogicalPlan::Extension(Extension { node }))
    } else {
        // existing clone + rebuild path
    }
}

Describe alternatives you've considered

A more general optimization would be to change UserDefinedLogicalNode::expressions() to return references (&[Expr]) instead of cloned Vec<Expr>, and only clone + rebuild when the transform actually modifies an expression. This would avoid the clone + with_exprs_and_inputs rebuild even for non-empty expression lists when the transform is a no-op. This would be a larger API change affecting all UserDefinedLogicalNode implementations, so the empty-expressions shortcut is a pragmatic first step.

Additional context

Benchmark results (criterion micro-benchmark):

Children	no_expr (optimized)	with_expr (rebuild)	Speedup
1	24 ns	167 ns	7x
3	23 ns	192 ns	8x
5	23 ns	181 ns	8x
10	24 ns	216 ns	9x

The no_expr path is constant time regardless of children count.