cgen: fix struct field with default optional value (fix #11119) (#22167) #6814

Job	Run time
tests-sanitize-address-clang	2h 3m 22s
tests-sanitize-address-gcc	2h 28m 54s
tests-sanitize-address-msvc	4m 4s
tests-sanitize-memory-clang	3h 10m 0s
tests-sanitize-undefined-clang	56m 24s
tests-sanitize-undefined-gcc	1h 28m 53s
	10h 11m 37s

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