Add quotes while printing const string value to support empty string

[rishabh413]

Below code:

import static com.networknt.schema.SpecVersion.VersionFlag.V202012;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.networknt.schema.JsonSchemaFactory;

String schema =
    "{"
        + "  \"type\": \"object\","
        + "  \"properties\": {"
        + "    \"myKey\" : {"
        + "        \"type\" : \"string\","
        + "        \"oneOf\" : ["
        + "          { \"const\": \"\" },"
        + "          { \"const\": \"a\" },"
        + "          { \"const\": \"b\" }"
        + "        ]"
        + "      }"
        + "  }"
        + "}";

String serializedData = "{ \"myKey\" : \"c\"}";
JsonNode data = new ObjectMapper().readTree(serializedData);

JsonSchemaFactory.getInstance(V202012)
    .getSchema(schema)
    .validate(data)
    .forEach(System.out::println);

gives below validation errors:

$.myKey: should be valid to one and only one schema, but 0 are valid
$.myKey: must be a constant value 
$.myKey: must be a constant value a
$.myKey: must be a constant value b

It would be very helpful if validation errors were:

$.myKey: should be valid to one and only one schema, but 0 are valid
$.myKey: must be a constant value ""
$.myKey: must be a constant value "a"
$.myKey: must be a constant value "b"

so that we can clearly see empty string.

I believe we need to change this line to:

const = {0}: must be a constant value "{1}"