Support blank identifiers, assume transient

[khvn26]

Contributes to #4278.

Evolve the API schemas as per the following:

POST /api/v1/identities (docs):

#/definitions/IdentifyWithTraits

{
    "required": [
        "identifier"
    ],
    "type": "object",
    "properties": {
        "identifier": {
            "title": "Identifier",
+           "x-nullable": true,
            "type": "string",
-           "minLength": 1
        },
        "traits": {
            "type": "array",
            "items": {
                "$ref": "#/definitions/TraitSerializerBasic"
            }
        },
        "flags": {
            "type": "array",
            "items": {
                "$ref": "#/definitions/SDKFeatureState"
            },
            "readOnly": true
         },
         "transient": {
             "type": "boolean",
             "x-nullable": true
        }
    }
}

If null/empty identifier provided, assume "transient": true. Supply identifier="" to the evaluation engine.