Fix `len` of empty Product sweep to match actual length #14905

Job	Run time
Build protos	9s
Coverage check	13m 11s
Changed files test	1m 12s
Doc test	2m 25s
Format check	7s
Lint check	2m 52s
Pytest MacOS (3.10)	10m 11s
Isolated pytest Ubuntu	8m 34s
Pytest MacOS (3.11)	6m 35s
Type check	2m 30s
Notebook formatting	16s
Notebook Tests against PR	10m 33s
Changed Notebooks Isolated Test against Cirq stable (partition-0)	45s
Packaging test	1m 18s
Changed Notebooks Isolated Test against Cirq stable (partition-1)	53s
Check consistency of requirements	30s
Pytest Ubuntu (3.10)	7m 57s
Changed Notebooks Isolated Test against Cirq stable (partition-2)	46s
Changed Notebooks Isolated Test against Cirq stable (partition-3)	52s
Pytest Ubuntu (3.11)	7m 3s
Misc check	10s
Shell check	7s
Bundle file consistency	46s
Typescript tests coverage	31s
Typescript lint check	32s
Typescript tests	40s
Pytest Windows (3.10)	14m 53s
Pytest Windows (3.11)	11m 24s
	1h 47m 42s

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