LibJS: Support array holes, encoded as empty JS::Value

This patch adds a new kind of JS::Value, the empty value.
It's what you get when you do JSValue() (or most commonly, {} in C++.)

An empty Value signifies the absence of a value, and should never be
visible to JavaScript itself. As of right now, it's used for array
holes and as a return value when an exception has been thrown and we
just want to unwind.

This patch is a bit of a mess as I had to fix a whole bunch of code
that was relying on JSValue() being undefined, etc.

Libraries/LibJS/Runtime/PropertyName.h

@@ -33,10 +33,13 @@ namespace JS {
 class PropertyName {
 public:
     enum class Type {
+        Invalid,
         Number,
         String,
     };
 
+    PropertyName() {}
+
     explicit PropertyName(i32 index)
         : m_type(Type::Number)
         , m_number(index)
@@ -50,6 +53,7 @@ public:
     {
     }
 
+    bool is_valid() const { return m_type != Type::Invalid; }
     bool is_number() const { return m_type == Type::Number; }
     bool is_string() const { return m_type == Type::String; }
 
@@ -57,7 +61,7 @@ public:
     const FlyString& as_string() const { return m_string; }
 
 private:
-    Type m_type;
+    Type m_type { Type::Invalid };
     FlyString m_string;
     i32 m_number { 0 };
 };