[script] fix when run dir is not found

Author: amin1377

vtr_flow/scripts/python_libs/vtr/util.py

@@ -540,12 +540,11 @@ def get_latest_run_dir(base_dir):
     """
     latest_run_dir_name = get_latest_run_dir_name(base_dir)
 
-    if latest_run_dir_name is None:
+    if latest_run_dir_name:
+        return str(PurePath(base_dir) / latest_run_dir_name)
+    else:
         return None
 
-    return str(PurePath(base_dir) / latest_run_dir_name)
-
-
 def get_existing_run_dir(base_dir: str, run_dir: str) -> str:
     """
     Get an existing run directory (from a previous run). Throw if it doesn't exist
@@ -585,7 +584,7 @@ def get_latest_run_dir_name(base_dir):
 
         if not run_dir.exists():
             # No existing run directories
-            return None
+            return ""
 
         while run_dir.exists():
             run_number += 1