Fix Parmatch crash on empty record head patterns #9551

Job	Run time
test-devcontainer	19m 47s
build-compiler (ubuntu-24.04, ocaml-variants.5.0.0+options,ocaml-option-static, linux-x64, x86_64...	7m 4s
build-compiler (macos-15-intel, 5.3.0, true, darwin-x64, x86_64-apple-darwin)	14m 33s
build-compiler (macos-15, 5.3.0, true, darwin-arm64, aarch64-apple-darwin)	9m 2s
build-compiler (ubuntu-24.04, ocaml-variants.5.3.0+options,ocaml-option-static, true, true, linux...	7m 52s
build-compiler (ubuntu-24.04-arm, ocaml-variants.5.3.0+options,ocaml-option-static, true, true, t...	8m 52s
build-compiler (windows-2025, 5.3.0, true, win32-x64, x86_64-pc-windows-gnu, .exe)	14m 45s
api-docs	8s
pkg-pr-new	54s
test-installation-pnpm (windows-2025)	31s
test-installation-npm (macos-15-intel)	36s
test-installation-pnpm (ubuntu-24.04-arm)	17s
test-installation-npm (macos-15)	16s
test-installation-pnpm (ubuntu-24.04)	11s
test-installation-npm (ubuntu-24.04)	13s
test-installation-pnpm (macos-15)	18s
test-integration-rewatch (macos-15-intel)	6m 46s
test-installation-pnpm (macos-15-intel)	28s
test-installation-npm (ubuntu-24.04-arm)	16s
test-integration-rewatch (windows-2025)	6m 46s
test-integration-rewatch (ubuntu-24.04-arm)	2m 58s
test-integration-rewatch (macos-15)	4m 14s
test-integration-rewatch (ubuntu-24.04)	3m 35s
test-installation-npm (windows-2025)	40s
publish	0s
	1h 51m 2s

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