LibJS: Let the VM cache an empty ("") PrimitiveString

Empty string is extremely common and we can avoid a lot of heap churn
by simply caching one in the VM. Primitive strings are immutable anyway
so there is no observable behavior change outside of fewer collections.

Libraries/LibJS/Runtime/PrimitiveString.cpp

@@ -41,6 +41,8 @@ PrimitiveString::~PrimitiveString()
 
 PrimitiveString* js_string(Heap& heap, String string)
 {
+    if (string.is_empty())
+        return &heap.vm().empty_string();
     return heap.allocate_without_global_object<PrimitiveString>(move(string));
 }