This is a follow up to https://github.com/swiftlang/swift/pull/80862, where `storeEnumTagSinglePayload` in a special implementation of `ResultTypeInfo` for Embedded Swift had a mismatching number of arguments. The actual declaration of it in `ABI/ValueWitness.def` clearly includes one more argument.
```
/// void (*storeEnumTagSinglePayload)(T* enum, UINT_TYPE whichCase,
/// UINT_TYPE emptyCases, M *self);
/// Given uninitialized memory for an instance of a single payload enum with a
/// payload of this witness table's type (e.g Optional<ThisType>), store the
/// tag.
FUNCTION_VALUE_WITNESS(storeEnumTagSinglePayload,
StoreEnumTagSinglePayload,
VOID_TYPE,
(MUTABLE_VALUE_TYPE, UINT_TYPE, UINT_TYPE, TYPE_TYPE))
```
This function type mismatch is illegal when targeting Wasm and traps at run time.
Similarly to #80862, we're passing `nullptr` as the newly added argument, which is equivalent to the existing behavior on other platforms.
rdar://157219474
