WasmLowering: Empty struct detection doesn't exist for ABI purposes, and should

The WebAssembly Basic C ABI (https://github.com/WebAssembly/tool-conventions/blob/main/BasicCABI.md) specifies that empty structs and unions are "ignored" for both parameters and returns. In WasmLowering.GetSignature, empty struct parameters are currently detected via
paramType.GetElementSize().AsInt == 0. However, .NET pads empty value types to a minimum size of 1 byte:

// MetadataFieldLayoutAlgorithm.cs:1177
// Pad the length of structs to be 1 if they are empty, so we have no zero-length structures
if (type.IsValueType && instanceSize == LayoutInt.Zero)
instanceSize = LayoutInt.One;

What needs to change

1. Detect empty structs by instance field count (0 non-static fields), no explicit layout.
2. Empty struct return/parameters should not contribute to the arguments of the function at the Wasm level.
3. This applies to all calling conventions (managed and unmanaged).

Location

src/coreclr/tools/Common/JitInterface/WasmLowering.cs — the GetSignature(MethodSignature, LoweringFlags) method.

从 src/coreclr/tools/Common/JitInterface/WasmLowering.cs 中的 GetSignature(MethodSignature, LoweringFlags) 开始，然后阅读 MetadataFieldLayoutAlgorithm.cs:1177 中提到的空值类型处理以及链接的 Basic C ABI 指南。完成标准是：对于托管和非托管调用约定，不包含非静态字段的空结构体（包括指定的显式布局情况）会从 WebAssembly 参数和返回值中省略。

由索引模型根据 Issue 内容生成。