$[$t],* ,$u accepted, but panics when $t is empty list

Prerequisites

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• Check that your issue is not already filed:
  https://github.com/leanprover/lean4/issues
• Reduce the issue to a minimal, self-contained, reproducible test case.
  Avoid dependencies to Mathlib or Batteries.
• Test your test case against the latest nightly release, for example on
  https://live.lean-lang.org/#project=lean-nightly
  (You can also use the settings there to switch to “Lean nightly”)

Description

In macro_rules, the (`tactic|...) quotation allows to concatenate a possibly empty list with a single element putting a separator in the middle. When the rule is triggered on an empty list, the result is a singleton list that starts with the separator, leading to some panic in tactic code. The following is a minimal example:

syntax "foo " ident,* : tactic

macro_rules
  | `(tactic| foo $[$terms],*) =>
   `(tactic| solve_by_elim only [$[$terms:ident],* ,0])

def foo: True := by
 foo

Steps to Reproduce

Run the code above. The last line triggers the panic

Versions

4.9.0-rc3

Start with the minimal macro_rules example in the issue and trace the tactic quotation handling for an empty $terms list. Done means the example no longer panics and produces the intended singleton list with the separator in the expected position.