Flatten pass can make invalid modules on tuple types

This came up while fuzzing, and I traced it using git bisect to PR #3710 where non-nullable locals are handled by handleNonNullableLocals. It doesn't seem to handle tuple types that contain a non-nullable ref, resulting in a validation error after Flatten pass.

seed: 17382324165229480416

Full validation result:

[wasm-validator error in function 3] unexpected false: vars must be defaultable, on 
(v128 (ref (func (param f32))) v128 i64)
[wasm-validator error in function 3] unexpected false: vars must be defaultable, on 
(v128 (ref (func (param f32))) v128 i64)
[wasm-validator error in function 3] unexpected false: vars must be defaultable, on 
(v128 (ref (func (param f32))) v128 i64)
[wasm-validator error in function 3] unexpected false: vars must be defaultable, on 
(v128 (ref (func (param f32))) v128 i64)
Fatal: Last pass (flatten) broke validation. Run with BINARYEN_PASS_DEBUG=2 in the env to see the earlier state, or 3 to dump byn-* files for each pass

Also wasm-reducer crashes with the following message when reducing the original file:

Missing type: (func (param externref f32) (result f32))
wasm-reduce: ../src/wasm/wasm-binary.cpp:529: uint32_t wasm::WasmBinaryWriter::getTypeIndex(wasm::HeapType) const: Assertion `0' failed.

Start by reproducing the reported seed through the Flatten pass, focusing on handleNonNullableLocals and tuple types containing a non-nullable reference. Use BINARYEN_PASS_DEBUG as suggested to inspect the earlier state and validate the result; done means Flatten no longer produces invalid modules and the reported wasm-reduce failure is addressed.