Fix #4041: Support zero-member types in NLOHMANN_DEFINE_TYPE_* macros

[suchetindrakanty]

Fixes #4041

This PR addresses an issue where NLOHMANN_DEFINE_TYPE_* macros fail when used with zero members.

Problem

When VA_ARGS is empty, macro expansion generates invalid code, causing compilation errors.

Solution

• Wrapped macro expansions with VA_OPT to ensure code is only generated when arguments are present
• Prevents invalid expansions for zero-member types
• Maintains existing behavior for non-empty cases

Testing

• Added test cases for intrusive and non-intrusive macros
• Covered WITH_DEFAULT and ONLY_SERIALIZE variants
• Verified empty structs serialize to {}
• Verified deserialization works correctly