Short-circuit on the count, not the array length, in common Encoding methods (dotnet...

Short-circuit on the count, not the array length, in common Encoding methods (dotnet/coreclr#6108)

* Short-circuit using count instead of array.Length
* Use Array.Empty in default Reset implementation

Commit migrated from https://github.com/dotnet/coreclr/commit/a1e785dccf35af4305f2536b1e25cb719e8f1943