LibJS: Don't die when making PrimitiveString from "" DeprecatedFlyString

Userland/Libraries/LibJS/Runtime/PrimitiveString.cpp

@@ -223,6 +223,8 @@ NonnullGCPtr<PrimitiveString> PrimitiveString::create(VM& vm, DeprecatedString s
 
 NonnullGCPtr<PrimitiveString> PrimitiveString::create(VM& vm, DeprecatedFlyString const& string)
 {
+    if (string.is_empty())
+        return vm.empty_string();
     return create(vm, *string.impl());
 }