Apply suggestions from code review

Co-authored-by: RubelMozumder <32923026+RubelMozumder@users.noreply.github.com>

Author: lukaspie

src/pynxtools/dataconverter/helpers.py

@@ -1193,8 +1193,6 @@ def clean_str_attr(
     Return the attribute as a string.
 
     - If `attr` is `bytes`, decode it using the given encoding.
-    - If `attr` is already a string, return it unchanged.
-    - If `attr` is `None`, return `None`.
     - Otherwise, return it unchanged.
 
     Args:
@@ -1204,8 +1202,6 @@ def clean_str_attr(
     Returns:
         The attribute as a string, or None if input was None.
     """
-    if attr is None or isinstance(attr, str):
-        return attr
     if isinstance(attr, bytes):
         return attr.decode(encoding)
     return attr

src/pynxtools/dataconverter/nexus_tree.py

@@ -363,7 +363,7 @@ def get_all_direct_children_names(
             tag_type = f"[@type='{nx_class}']"
 
         if node_type is not None:
-            search_tags = f"*[self::nx:{node_type}{tag_type}]"
+            search_tags = rf"*[self::nx:{node_type}{tag_type}]"
         else:
             search_tags = (
                 r"*[self::nx:field or self::nx:group "