Handle the empty strings as a group name

holoviews/core/util.py

@@ -346,6 +346,8 @@ def tree_attribute(identifier):
     These custom attributes start with a capitalized character when
     applicable (not applicable to underscore or certain unicode characters)
     """
+    if not identifier:
+        return True
     if identifier[0].upper().isupper() is False and identifier[0] != '_':
         return True
     else:
@@ -1750,7 +1752,10 @@ def capitalize(string):
     """
     Capitalizes the first letter of a string.
     """
-    return string[0].upper() + string[1:]
+    if string:
+        return string[0].upper() + string[1:]
+    else:
+        return string
 
 
 def get_path(item):