[red-knot] Fix merged type after if-else without explicit else branch #36474

Job	Run time
cargo fmt	16s
cargo build (release)	0s
mkdocs	2m 25s
Determine changes	9s
pre-commit	44s
python package	1m 26s
cargo test (windows)	6m 9s
cargo clippy	2m 19s
cargo test (linux, release)	3m 34s
cargo build (msrv)	4m 3s
benchmarks	4m 45s
cargo fuzz build	0s
cargo shear	53s
cargo test (linux)	1m 48s
cargo test (wasm)	1m 38s
test scripts	1m 3s
formatter instabilities and black similarity	0s
ecosystem	2m 44s
test ruff-lsp	40s
fuzz parser	0s
	34m 36s

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