[bugfix] Fix ForClause PositionalVar empty sequence handling.

Author: Robin6939

exist-core/src/main/java/org/exist/xquery/ForExpr.java

@@ -235,7 +235,8 @@ private void processItem(LocalVariable var, Item contextItem, Sequence in, Seque
         context.proceed(this);
         context.setContextSequencePosition(p, in);
         if (positionalVariable != null) {
-            at.setValue(new IntegerValue(this, p + 1));
+            final int position = contextItem == AtomicValue.EMPTY_VALUE ? 0 : p + 1;
+            at.setValue(new IntegerValue(this, position));
         }
         final Sequence contextSequence = contextItem.toSequence();
         // set variable value to current item

exist-core/src/test/xquery/xquery3/flwor.xql

@@ -156,3 +156,32 @@ function flwor:allowing-empty($n as xs:integer) {
     return concat("[", $x, "]")
 };
 
+declare
+    %test:args(4)
+    %test:assertEquals(":0")
+    %test:args(2)
+    %test:assertEquals("b:1")
+    %test:args(1)
+    %test:assertEquals("a:1")
+    %test:args(5)
+    %test:assertEquals(":0")
+function flwor:allowing-empty-fix($n as xs:integer) {
+    let $sequence := ("a", "b", "c")[$n]
+    for $x allowing empty at $y in $sequence
+    return $x || ":" || $y
+};
+
+declare
+    %test:args(4)
+    %test:assertEquals("")
+    %test:args(2)
+    %test:assertEquals("b:1")
+function flwor:no-allow-empty($n as xs:integer) {
+    let $sequence := ("a", "b", "c")[$n]
+    return
+        if (empty($sequence)) then
+            ""
+        else
+            for $x at $y in $sequence
+            return $x || ":" || $y
+};