panic in `autocomplete` when a zero-length completion rule is passed

[lmorg]

Describe the bug:

murex-dev» autocomplete set bob %[]
Error: runtime error: index out of range [0] with length 0
Stack:

  - function: runtime.gopanic(...)
    filename: /opt/homebrew/Cellar/go/1.25.3/libexec/src/runtime/panic.go:783

  - function: runtime.goPanicIndex(...)
    filename: /opt/homebrew/Cellar/go/1.25.3/libexec/src/runtime/panic.go:115

  - function: github.com/lmorg/murex/builtins/core/autocomplete.set(...)
    filename: /Users/laurencemorgan/dev/go/src/github.com/lmorg/murex/builtins/core/autocomplete/autocomplete.go:97

  - function: github.com/lmorg/murex/builtins/core/autocomplete.cmdAutocomplete(...)
    filename: /Users/laurencemorgan/dev/go/src/github.com/lmorg/murex/builtins/core/autocomplete/autocomplete.go:29

  - function: github.com/lmorg/murex/lang.executeProcess(...)
    filename: /Users/laurencemorgan/dev/go/src/github.com/lmorg/murex/lang/process.go:332
Build:
  OS:  darwin
  CPU: arm64
Compiler: go1.25.3
---

!!! Murex has crashed !!!