[mlir][SparseTensor] Fix type conversion rule (#140350)
A type conversion rule cannot make any assumptions about the number of
pre-existing types in the `results` vector.
This commit fixes a failed assertion in a SparseTensor type conversion
rule. This is only reproducible when type conversion caching is
deactivated. There's no way to do this at the moment. This commit is in
preparation of adding context-aware type conversions, which will
deactivate type caching in such cases.
