[bugfix] Don't allow xs:string to be cast as xs:QName.

Fix #3390

Author: rhubner

exist-core/src/main/java/org/exist/xquery/value/StringValue.java

@@ -512,8 +512,6 @@ public AtomicValue convertTo(int requiredType) throws XPathException {
                 return new GMonthDayValue(value);
             case Type.UNTYPED_ATOMIC:
                 return new UntypedAtomicValue(getStringValue());
-            case Type.QNAME:
-                return new QNameValue(null, new QName(value, XMLConstants.NULL_NS_URI));
             default:
                 throw new XPathException(ErrorCodes.FORG0001, "cannot cast '" +
                         Type.getTypeName(this.getItemType()) + "(\"" + getStringValue() + "\")' to " +

exist-core/src/test/xquery/xquery3/castable.xqm

@@ -0,0 +1,11 @@
+xquery version "3.1";
+
+module namespace t="http://exist-db.org/xquery/test";
+
+declare namespace test="http://exist-db.org/xquery/xqsuite";
+
+declare
+    %test:assertFalse
+function t:castable-test() {
+    "0" castable as xs:QName
+};