JSON.parse("") causes follow-up errors

[Holger Knublauch]

Test case:

import org.apache.jena.atlas.json.JSON;
import org.junit.Test;
public class JSONTest {

    @Test
    public void testRepeat() {
        try {
            JSON.parse("");
        }
        catch(Exception ex) {
            // Ignore
            System.out.println("Expected error " + ex);
        }
        JSON.parse("{}");
    }
}

works fine if the first JSON.parse gets commented out, but reports

org.apache.jena.atlas.json.JsonParseException: Not a JSON object START: 
[null]
    at 
org.apache.jena.atlas.json.io.parser.ParserBase.exception(ParserBase.java:130)
    at org.apache.jena.atlas.json.io.parser.JSONP.parse(JSONP.java:49)
    at 
org.apache.jena.atlas.json.io.parser.JSONParser.parse(JSONParser.java:58)
    at 
org.apache.jena.atlas.json.io.parser.JSONParser.parse(JSONParser.java:52)
    at org.apache.jena.atlas.json.JSON._parse(JSON.java:125)
    at org.apache.jena.atlas.json.JSON._parse(JSON.java:105)
    at org.apache.jena.atlas.json.JSON.parse(JSON.java:44)
    at org.topbraid.jenax.test.JSONTest.testRepeat(JSONTest.java:17)

otherwise. The first parsing should not have an effect on subsequent 
calls. Our work-around is not to ever call JSON.parse with an empty 
string, so it's not high priority but I thought I'd report it anyway in 
case there is a deeper problem (e.g. with concurrency of this class).

Thanks
Holger