Change MySqlConnectionStringBuilder this[string] to return object #837
Description
MySqlConnectionStringBuilder should return object, not object?, from this[string keyword]. All reference properties should return non-null objects, e.g., an empty string instead of null.
Also add [AllowNull].
This is based on the most recent null-annotation proposal at https://github.com/dotnet/runtime/pull/689/files#diff-fbc8b4fa5b891e40cb60fd8fd075cc81R1964-R1965.