fix(@angular/pwa): support using default index option when not present #7782

Job	Run time
lint	2m 18s
build	3m 25s
e2e_windows (windows-2025, 22, npm, 0)	5m 53s
e2e_windows (windows-2025, 22, npm, 1)	5m 52s
e2e_windows (windows-2025, 22, npm, 2)	5m 54s
e2e_windows (windows-2025, 22, npm, 3)	5m 55s
e2e_windows (windows-2025, 22, npm, 4)	5m 54s
e2e_windows (windows-2025, 22, npm, 5)	5m 52s
e2e_windows (windows-2025, 22, esbuild, 0)	5m 55s
e2e_windows (windows-2025, 22, esbuild, 1)	5m 48s
e2e_windows (windows-2025, 22, esbuild, 2)	5m 48s
e2e_windows (windows-2025, 22, esbuild, 3)	5m 48s
e2e_windows (windows-2025, 22, esbuild, 4)	5m 54s
e2e_windows (windows-2025, 22, esbuild, 5)	5m 55s
test	2m 36s
publish-snapshots	2m 36s
Browser Compatibility Tests	2m 35s
e2e-package-managers	1s
e2e	1s
e2e-snapshots	1s
	1h 24m 1s

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