[bugfix] Small cleanup to REST Server External Variable handling

Author: adamretter

exist-core/src/main/java/org/exist/http/RESTServer.java

@@ -1500,21 +1500,25 @@ private void declareExternalAndXQJVariables(final XQueryContext context,
                 child = (NodeImpl) child.getNextSibling();
             }
 
-            if (uri != null && prefix != null) {
-                context.declareNamespace(prefix, uri);
+            if (localname == null) {
+                throw new XPathException(ErrorCodes.W3CErrorCode.XPDY0002, String.format("External variable is missing local name in its qualified name. Prefix=%s URI=%s", prefix, uri));
             }
 
-            if (localname == null) {
-                continue;
+            if (uri == null && prefix != null) {
+                uri = context.getURIForPrefix(prefix);
             }
 
             final QName q;
-            if (prefix != null && localname != null) {
+            if (prefix != null) {
                 q = new QName(localname, uri, prefix);
             } else {
                 q = new QName(localname, uri, XMLConstants.DEFAULT_NS_PREFIX);
             }
 
+            if (uri != null && prefix != null) {
+                context.declareNamespace(prefix, uri);
+            }
+
             // get serialized sequence
             final NodeImpl value = variable.getFirstChild(new NameTest(Type.ELEMENT, Marshaller.SEQUENCE_ELEMENT_QNAME));
             final Sequence sequence;