Fix: .NET Standard 2.0 Shims unsafe span overloads

[ivan-prodanov]

Summary

Fix shims for .NET Standard 2.0: correct pointer-based Encoding overload usage and add guards for empty spans.

Problem

GetBytes/GetString used span pinning paths that misbehave on empty spans under .NET Standard 2.0. Problem occurs when logging empty string with exception

logger.Log(LogLevel.Error, new InvalidOperationException("WTF"), string.Empty);

The line above is logged in .NET 8 clients, but not in .net framework clients - the Shims throw IndexOutOfRange exception because the message is empty, but the shim attempts to get first character.

Fix

• Added early-return guards for zero-length inputs.
• Used fixed-pointer overloads to call Encoding.GetBytes/GetString safely.

Tests

• Modified unit tests to run both under net framework and net core.
• Added unit tests covering empty spans, non-empty spans, and boundary sizes.
• All existing tests pass.