db: keep NOT NULL constraint on is_immature in migration #1050

Job	Run time
linter_gui	4m 6s
linter	47s
unit_tests (1.54, ubuntu-latest)	3m 6s
unit_tests (1.54, macOS-latest)	8m 29s
unit_tests (1.54, windows-latest)	5m 10s
unit_tests (nightly, ubuntu-latest)	1m 19s
unit_tests (nightly, macOS-latest)	1m 59s
unit_tests (nightly, windows-latest)	2m 0s
unit_tests_gui (1.65, ubuntu-latest)	5m 51s
unit_tests_gui (1.65, macOS-latest)	6m 18s
unit_tests_gui (1.65, windows-latest)	9m 4s
unit_tests_gui (nightly, ubuntu-latest)	4m 13s
unit_tests_gui (nightly, macOS-latest)	3m 48s
unit_tests_gui (nightly, windows-latest)	7m 8s
	1h 3m 18s

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