[bugfix] allow empty options map in map:merge#2

fixes #4620

Check for existence of "duplicates" option first.
Add test to ensure map:merge#2 with empty options behaves as
map:merge#1.

Author: line-o

exist-core/src/main/java/org/exist/xquery/functions/map/MapFunction.java

@@ -250,8 +250,10 @@ private Sequence merge(final Sequence[] args) throws XPathException {
 
         final MergeDuplicates mergeDuplicates;
         if (args.length == 2) {
-            final Sequence mapValue = ((MapType) args[1]).get(new StringValue(this, "duplicates"));
-            if (mapValue != null) {
+            final MapType map = (MapType) args[1];
+            final StringValue key = new StringValue(this, "duplicates");
+            if (map.contains(key)) {
+                final Sequence mapValue = map.get(key);
                 mergeDuplicates = MergeDuplicates.fromDuplicatesValue(mapValue.getStringValue());
                 if (mergeDuplicates == null) {
                     throw new XPathException(this, ErrorCodes.FOJS0005, "value for duplicates key was not recognised: " + mapValue.getStringValue());

exist-core/src/test/xquery/maps/maps.xqm

@@ -976,3 +976,16 @@ function mt:immutable-merge-duplicates-then-merge() {
             $expected ne $result($mt:test-key-one)
         )
 };
+
+(:~
+ : ensure that empty options map is allowed and behaves like
+ : map:merge#1
+ :)
+declare
+    %test:assertTrue
+function mt:map-merge-2-empty-options-map() {
+    let $maps := (mt:getMapFixture(), map { "Su": "Sunnuntai" })
+    let $expected := map:merge($maps)
+    let $actual := map:merge($maps, map {})
+    return $expected?Su eq $actual?Su
+};