`ref.null` produces wrong result in iwasm fast-interp and llvm-jit backends

[akldc]

Test case

(module
  (type $0 (func (result (ref null i31) i32)))
  (export "main" (func 0))
  (func $0
    (type 0)
    (ref.null i31)
    (i32.const 1)
  )
)

Describe the bug
When running this module with iwasm:

$ iwasm --interp -f main test1.wasm
any:ref.null,0x1:i32

$ iwasm -f main test1.wasm
ref.i31,0x0:i32

$ iwasm --llvm-jit -f main test1.wasm
ref.i31,0x0:i32

The result is correct in interp mode, but incorrect in fast-interp and llvm-jit modes.
In fast-interp and llvm-jit modes, the typed null value produced by ref.null i31 is incorrectly treated as a concrete i31 reference with value 0. This not only changes the reference kind (null vs non-null), but also corrupts the following stack layout, causing the subsequent i32.const 1 to be misinterpreted and observed as 0x0 instead of 0x1.

Version
iwasm 2.4.4
ubuntu 20.04

To Reproduce
Steps to reproduce the behavior:

1. Compile iwasm with flags like '-DWAMR_BUILD_JIT=1 -DWAMR_BUILD_TAIL_CALL=1 -DWAMR_BUILD_GC=1'
2. Run iwasm with CLI options like 'iwasm -f main test1.wasm'
3. See error

Expected behavior
any:ref.null,0x1:i32

Actual Result
ref.i31,0x0:i32

Desktop (please complete the following information):

• Arch x86_64
• OS ubuntu 20.04