Small simplification in Screen code.

See also: #1877

src/prompt_toolkit/layout/screen.py

@@ -169,7 +169,7 @@ def __init__(
 
         #: Escape sequences to be injected.
         self.zero_width_escapes: defaultdict[int, defaultdict[int, str]] = defaultdict(
-            lambda: defaultdict(lambda: "")
+            lambda: defaultdict(str)
         )
 
         #: Position of the cursor.