Simplify nil-handler guards in polyscript.go and engines/*/new.go (after #111)

[robbyt]

Summary

After #111 lands (relaxing WithLogHandler(nil) and WithLogger(nil) to no-op across the three engines' compiler packages), six call sites that exist solely to work around the current nil-rejection become redundant:

File:line	Current pattern
polyscript.go:251-253	if cfg.handler != nil { opts = append(opts, risorMachine.WithLogHandler(cfg.handler)) }
polyscript.go:262-264	same shape, starlarkMachine.WithLogHandler
polyscript.go:273-275	same shape, extismMachine.WithLogHandler
engines/risor/new.go:72-74	if cfg.handler != nil { compilerOpts = append(..., compiler.WithLogHandler(cfg.handler)) }
engines/starlark/new.go:72-74	same shape
engines/extism/new.go:85-87	same shape

Once the underlying option is itself a no-op on nil, the guards do nothing useful — the unconditional append produces identical behavior.

Proposal

Drop all six guards. Each pair of lines collapses to a single unconditional append:

opts = append(opts, risorMachine.WithLogHandler(cfg.handler))

The cfg.handler field is plain slog.Handler whose zero value is nil; the option now handles that correctly. No tests should need updating — the existing "nil handler is fine" tests stay green either way.

Out of scope

• Behavior change for non-nil handlers — unchanged.
• Other defensive nil-checks elsewhere in the codebase — only the six listed above are downstream of this specific design tension.

Files

• polyscript.go — three sites
• engines/risor/new.go
• engines/starlark/new.go
• engines/extism/new.go

Depends on

• Asymmetric nil-handler behavior: WithLogHandler errors on nil while New()/newEvalResult inherit from slog.Default() #111 — the option-side relaxation. This issue should not be implemented until Asymmetric nil-handler behavior: WithLogHandler errors on nil while New()/newEvalResult inherit from slog.Default() #111 is merged; until then, dropping the guards would cause WithLogHandler(nil) to return an error during normal construction with no WithLogHandler option supplied.