tuple_extract: correctly extract key from tuples without trailing nils

Function tuple_extract_key_raw for nullable partially sequential key_def
can extract wrong key from tuples without trailing nils. Fortunately,
it does not affect tarantool - tuple_extract_key_raw is used only with
cmp_def, which ends with non-nullable primary key definition, hence
contains no trailing nils.

Closes #8504

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