Add leniency to missing array values in mustache (#126550)

In mustache, this change returns null values which convert to empty strings 
instead of throwing an exception when users have a template with 
something like a.8 where the index 8 is out of bounds. This matches the 
behavior for non-existent keys like a.d.

Closes #55200

Author: jdconrad

docs/changelog/126550.yaml

@@ -0,0 +1,6 @@
+pr: 126550
+summary: Add leniency to missing array values in mustache
+area: Infra/Scripting
+type: bug
+issues:
+ - 55200

modules/lang-mustache/src/main/java/org/elasticsearch/script/mustache/CustomReflectionObjectHandler.java

@@ -111,11 +111,11 @@ public Object get(Object key) {
             if ("size".equals(key)) {
                 return size();
             } else if (key instanceof Number number) {
-                return Array.get(array, number.intValue());
+                return number.intValue() >= 0 && number.intValue() < length ? Array.get(array, number.intValue()) : null;
             }
             try {
                 int index = Integer.parseInt(key.toString());
-                return Array.get(array, index);
+                return index >= 0 && index < length ? Array.get(array, index) : null;
             } catch (NumberFormatException nfe) {
                 // if it's not a number it is as if the key doesn't exist
                 return null;
@@ -169,11 +169,11 @@ public Object get(Object key) {
             if ("size".equals(key)) {
                 return col.size();
             } else if (key instanceof Number number) {
-                return Iterables.get(col, number.intValue());
+                return number.intValue() >= 0 && number.intValue() < col.size() ? Iterables.get(col, number.intValue()) : null;
             }
             try {
                 int index = Integer.parseInt(key.toString());
-                return Iterables.get(col, index);
+                return index >= 0 && index < col.size() ? Iterables.get(col, index) : null;
             } catch (NumberFormatException nfe) {
                 // if it's not a number it is as if the key doesn't exist
                 return null;

modules/lang-mustache/src/test/java/org/elasticsearch/script/mustache/MustacheTests.java

@@ -141,6 +141,26 @@ public void testMapInArrayAccess() throws Exception {
         assertThat(output, both(containsString("foo")).and(containsString("bar")));
     }
 
+    public void testMapInArrayBadAccess() throws Exception {
+        String template = "{{data.0.key}} {{data.2.key}}";
+        TemplateScript.Factory factory = engine.compile(null, template, TemplateScript.CONTEXT, Collections.emptyMap());
+        Map<String, Object> vars = new HashMap<>();
+        vars.put("data", new Object[] { singletonMap("key", "foo"), singletonMap("key", "bar") });
+        // assertThat(factory.newInstance(vars).execute(), equalTo("foo "));
+
+        vars.put("data", Arrays.asList(singletonMap("key", "foo"), singletonMap("key", "bar")));
+        factory.newInstance(vars);
+        assertThat(factory.newInstance(vars).execute(), equalTo("foo "));
+
+        // HashSet iteration order isn't fixed
+        Set<Object> setData = new HashSet<>();
+        setData.add(singletonMap("key", "foo"));
+        setData.add(singletonMap("key", "bar"));
+        vars.put("data", setData);
+        String output = factory.newInstance(vars).execute();
+        assertThat(output, both(containsString("foo")).and(not(containsString("bar"))));
+    }
+
     public void testSizeAccessForCollectionsAndArrays() throws Exception {
         String[] randomArrayValues = generateRandomStringArray(10, 20, false);
         List<String> randomList = Arrays.asList(generateRandomStringArray(10, 20, false));