emtpy syntax tree case

src/ytdl_sub/script/script.py

@@ -1,5 +1,4 @@
 # pylint: disable=missing-raises-doc
-import copy
 from typing import Dict
 from typing import List
 from typing import Optional

src/ytdl_sub/script/types/syntax_tree.py

@@ -55,6 +55,10 @@ class SyntaxTree(VariableDependency):
             custom_functions=custom_functions,
         )
 
+        # If no arguments, must be empty string
+        if len(maybe_resolvable_values) == 0:
+            return String(value="")
+
         # Mimic the above resolve behavior
         if len(maybe_resolvable_values) > 1:
             return BuiltInFunction(name="concat", args=maybe_resolvable_values)