Anonymous lambda should have empty function name (#1024)

Author: lahma

Jint.Tests/Runtime/FunctionTests.cs

@@ -94,5 +94,12 @@ function test() {
                 .SetValue("assertEqual", new Action<object, object>((a, b) => Assert.Equal(b, a)))
                 .Execute(script);
         }
+
+        [Fact]
+        public void AnonymousLambdaShouldHaveNameDefined()
+        {
+            var engine = new Engine();
+            Assert.True(engine.Evaluate("(()=>{}).hasOwnProperty('name')").AsBoolean());
+        }
     }
 }

Jint/Native/Function/FunctionInstance.cs

@@ -213,7 +213,7 @@ public override void RemoveOwnProperty(JsValue property)
 
         internal void SetFunctionName(JsValue name, string prefix = null, bool force = false)
         {
-            if (!force && _nameDescriptor != null && !UnwrapJsValue(_nameDescriptor).IsUndefined())
+            if (!force && _nameDescriptor != null && UnwrapJsValue(_nameDescriptor) != JsString.Empty)
             {
                 return;
             }

Jint/Runtime/Interpreter/Expressions/JintArrowFunctionExpression.cs

@@ -1,4 +1,5 @@
 using Esprima.Ast;
+using Jint.Native;
 using Jint.Native.Function;
 
 namespace Jint.Runtime.Interpreter.Expressions
@@ -25,6 +26,11 @@ protected override ExpressionResult EvaluateInternal(EvaluationContext context)
                 FunctionThisMode.Lexical,
                 proto: engine.Realm.Intrinsics.Function.PrototypeObject);
 
+            if (_function.Name is null)
+            {
+                closure.SetFunctionName(JsString.Empty);
+            }
+
             return NormalCompletion(closure);
         }
     }