fix: Platform calling-convention getters return null instead of throwing

[tinysec]

Summary

DefaultCallingConvention, Cdecl/Stdcall/FastcallCallingConvention, and SystemCallConvention wrapped their handle with CallingConvention.MustTakeHandle, which throws ArgumentNullException when the core returns a null handle. Python's default_calling_convention (and the four siblings) return None for an unset convention.

Switch each getter to CallingConvention.TakeHandle (same owner=true ownership — BNGetPlatform* returns an owned ref — but returns null on IntPtr.Zero) and widen the return type to CallingConvention?. Mirrors Architecture.GetCdeclCallingConvention which already does the null-safe thing via NewFromHandle.

Verification

• Binding builds clean (0 warnings, 0 errors).
• New E2E test: the default convention on linux-x86_64 is "sysv"; reaching .Name proves the handle is passed through TakeHandle with ownership intact. All five getters are reachable without throwing.

Note: every standard Binary Ninja platform defines all five calling conventions, so no fixture platform yields a natural null. The no-throw-on-null contract is guaranteed by TakeHandle (returns null on IntPtr.Zero); the test verifies the real-world non-null path.