Setting connection string builder property to null should remove it

Execute the following code:

```csharp
var csb = new MySqlConnectionStringBuilder();
csb.Server = "abc";
csb.Server = null;
csb.UserID = "abc";
csb.UserID = null;
Console.WriteLine(csb.ConnectionString);
```

Expected output is `""`; actual output is `"Server=;User Id="`.

Via https://github.com/dotnet/runtime/pull/689.