Added dependency for the Nashorn JS engine which was removed in Java 15

Author: bithazard

handlebars/pom.xml

@@ -173,6 +173,13 @@
       <optional>true</optional>
     </dependency>
 
+    <!-- Nashorn JS engine -->
+    <dependency>
+      <groupId>org.openjdk.nashorn</groupId>
+      <artifactId>nashorn-core</artifactId>
+      <version>15.3</version>
+    </dependency>
+
     <!-- Logging System -->
     <dependency>
       <groupId>org.slf4j</groupId>

handlebars/src/main/java/com/github/jknack/handlebars/Handlebars.java

@@ -1474,7 +1474,7 @@ private ScriptEngine engine() {
     synchronized (this) {
       if (this.engine == null) {
 
-        this.engine = new ScriptEngineManager(null).getEngineByName("nashorn");
+        this.engine = new ScriptEngineManager().getEngineByName("nashorn");
 
         Throwing.run(() -> engine.eval(Files.read(this.handlebarsJsFile, charset)));
       }