add nullable serialization to flags converters

[sergei-boguslavski]

Microsoft.SqlTools.Hosting.v2 uses data contracts that have nullable [Flags] enums. For example:

sqltoolsservice/src/Microsoft.SqlTools.DataProtocol.Contracts/ClientCapabilities/Workspace/Symbol.cs

Line 35 in 361287a

public SymbolKinds? ValueSet { get; set; }

. It is supposed to be restored from this json which was copy-pasted from the "initialize" request sent by sqlops data client:

"symbol": {
  "dynamicRegistration": true,
    "symbolKind": {
        "valueSet": [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26]
    }
},

However, this request causes exception in the serialization code because it does not handle the nullables:

sqltoolsservice/src/Microsoft.SqlTools.DataProtocol.Contracts/Utilities/FlagsIntConverter.cs

Line 29 in 361287a

// TODO: Fix to handle nullables properly

The proposed change fixes the deserialization issue.

[msftclas]

All CLA requirements met.

[coveralls]

Coverage decreased (-0.03%) to 76.756% when pulling 1354a4c on sergei-boguslavski:master into 55c82fb on microsoft:master.