Function valueToString

docs/app/js/sanddance-app.js:6210–6220

(value)

    return !!value;
}
function valueToString(value) {
    if (value == null) return "";
    switch(typeof value){
        case "string":
            return value;
        case "boolean":
        case "number":
            return value.toString();
    }
    return "";
}
function isStringOperation(ex) {
    switch(ex.operator){
        case "contains":