avoid crashing when there is no _children

[JoviDeCroock]

Similar to https://github.com/preactjs/preact/blob/main/debug/src/debug.js#L327

Fixes #4146

[coveralls]

coverage: 99.293% (-0.001%) from 99.294% when pulling ebf45ef on avoid-debug-crash into 08b07cc on main.

[github-actions]

Size Change: +15 B (0%)

Total Size: 57.3 kB

Filename	Size	Change
debug/dist/debug.js	3.54 kB	+5 B (0%)
debug/dist/debug.module.js	3.53 kB	+6 B (0%)
debug/dist/debug.umd.js	3.62 kB	+4 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
compat/dist/compat.js	3.95 kB	0 B
compat/dist/compat.module.js	3.87 kB	0 B
compat/dist/compat.umd.js	4.01 kB	0 B
devtools/dist/devtools.js	231 B	0 B
devtools/dist/devtools.module.js	240 B	0 B
devtools/dist/devtools.umd.js	315 B	0 B
dist/preact.js	4.42 kB	0 B
dist/preact.min.js	4.45 kB	0 B
dist/preact.min.module.js	4.45 kB	0 B
dist/preact.min.umd.js	4.47 kB	0 B
dist/preact.module.js	4.45 kB	0 B
dist/preact.umd.js	4.48 kB	0 B
hooks/dist/hooks.js	1.53 kB	0 B
hooks/dist/hooks.module.js	1.56 kB	0 B
hooks/dist/hooks.umd.js	1.62 kB	0 B
jsx-runtime/dist/jsxRuntime.js	360 B	0 B
jsx-runtime/dist/jsxRuntime.module.js	326 B	0 B
jsx-runtime/dist/jsxRuntime.umd.js	441 B	0 B
test-utils/dist/testUtils.js	453 B	0 B
test-utils/dist/testUtils.module.js	454 B	0 B
test-utils/dist/testUtils.umd.js	536 B	0 B

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