[SPIR-V] Compiler crash when passing empty ConstantBuffer to a function

Description
Compiler crashes on null pointer access when using empty struct.

Steps to Reproduce
command line:

dxc /nologo /Od /Zi -spirv -fspv-flatten-resource-arrays -Ecrash -Tvs_6_0 -Fo crash.spv crash.hlsl

crash.hlsl:

struct L { float2 p0, p1; };
struct U {};

static inline L crashme(U u) {
    L o;

    o.p0 = float2(0, 0);
    o.p1 = float2(0, 1);

    return o;
}

[[ vk::binding(0, 0) ]] ConstantBuffer<U> us : register(b0, space0);

struct O {
    float4 p : SV_POSITION;
};

O crash() {

    // crash
    L pl = crashme(us);

    O o;
    o.p = float4(0,0,0,1);

    return o;
}

Actual Behavior
crash on null pointer:

libdxcompiler.dylib`clang::spirv::SpirvEmitter::generateFromScalars:
->  0x1029c90b8 <+76>: ldr    x8, [x8]
    0x1029c90bc <+80>: ldr    w8, [x8, #0x1c]
    0x1029c90c0 <+84>: str    x8, [sp, #0x28]
    0x1029c90c4 <+88>: mov    x0, x1
Target 0: (dxc) stopped.
(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
  * frame #0: 0x00000001029c90b8 libdxcompiler.dylib`clang::spirv::SpirvEmitter::generateFromScalars(clang::QualType, std::__1::vector<clang::spirv::SpirvInstruction*, std::__1::allocator<clang::spirv::SpirvInstruction*>>&, clang::spirv::SpirvLayoutRule) + 76
    frame #1: 0x00000001029c873c libdxcompiler.dylib`clang::spirv::SpirvEmitter::processFlatConversion(clang::QualType, clang::spirv::SpirvInstruction*, clang::SourceLocation, clang::SourceRange) + 228
    frame #2: 0x00000001029b83bc libdxcompiler.dylib`clang::spirv::SpirvEmitter::doCastExpr(clang::CastExpr const*, clang::SourceRange) + 3384
    frame #3: 0x00000001029c37cc libdxcompiler.dylib`clang::spirv::SpirvEmitter::processCall(clang::CallExpr const*) + 1416
    frame #4: 0x00000001029bb2e8 libdxcompiler.dylib`clang::spirv::SpirvEmitter::loadIfGLValue(clang::Expr const*, clang::SourceRange) + 72
    frame #5: 0x00000001029b2624 libdxcompiler.dylib`clang::spirv::SpirvEmitter::doVarDecl(clang::VarDecl const*) + 2104
    frame #6: 0x00000001029b4830 libdxcompiler.dylib`clang::spirv::SpirvEmitter::doStmt(clang::Stmt const*, llvm::ArrayRef<clang::Attr const*>) + 1496
    frame #7: 0x00000001029b43c0 libdxcompiler.dylib`clang::spirv::SpirvEmitter::doStmt(clang::Stmt const*, llvm::ArrayRef<clang::Attr const*>) + 360
    frame #8: 0x00000001029b37f4 libdxcompiler.dylib`clang::spirv::SpirvEmitter::doFunctionDecl(clang::FunctionDecl const*) + 3932
    frame #9: 0x00000001029ae8c8 libdxcompiler.dylib`clang::spirv::SpirvEmitter::HandleTranslationUnit(clang::ASTContext&) + 964
    frame #10: 0x0000000102a2b2a0 libdxcompiler.dylib`clang::ParseAST(clang::Sema&, bool, bool) + 496
    frame #11: 0x0000000102348208 libdxcompiler.dylib`clang::FrontendAction::Execute() + 100
    frame #12: 0x0000000101c440bc libdxcompiler.dylib`DxcCompiler::Compile(DxcBuffer const*, wchar_t const**, unsigned int, IDxcIncludeHandler*, GUID const&, void**) + 7060
    frame #13: 0x0000000101c4004c libdxcompiler.dylib`hlsl::DxcCompilerAdapter::WrapCompile(bool, IDxcBlob*, wchar_t const*, wchar_t const*, wchar_t const*, wchar_t const**, unsigned int, DxcDefine const*, unsigned int, IDxcIncludeHandler*, IDxcOperationResult**, wchar_t**, IDxcBlob**) + 1252
    frame #14: 0x0000000101c41000 libdxcompiler.dylib`hlsl::DxcCompilerAdapter::CompileWithDebug(IDxcBlob*, wchar_t const*, wchar_t const*, wchar_t const*, wchar_t const**, unsigned int, DxcDefine const*, unsigned int, IDxcIncludeHandler*, IDxcOperationResult**, wchar_t**, IDxcBlob**) + 156
    frame #15: 0x0000000101c418e8 libdxcompiler.dylib`hlsl::DxcCompilerAdapter::Compile(IDxcBlob*, wchar_t const*, wchar_t const*, wchar_t const*, wchar_t const**, unsigned int, DxcDefine const*, unsigned int, IDxcIncludeHandler*, IDxcOperationResult**) + 44
    frame #16: 0x00000001000204d0 dxc`DxcContext::Compile() + 2352
    frame #17: 0x000000010002429c dxc`dxc::main(int, char const**) + 2208
    frame #18: 0x0000000189593da4 dyld`start + 6992

Environment

• DXC version [(libdxcompiler.dylib: 1.10(5304-0131a626)(1.9.0.15304))]
• Host Operating System osx 26.4

Start with the provided crash.hlsl reproduction and the SPIR-V emitter stack trace, especially SpirvEmitter::generateFromScalars and its caller processFlatConversion. Run the listed dxc command to confirm the null-pointer crash, then verify that compiling the empty ConstantBuffer case completes and emits SPIR-V without crashing.