PERF: Add if branch for empty sep in str.cat

pandas/core/strings.py

@@ -48,6 +48,8 @@ def cat_core(list_of_columns, sep):
     nd.array
         The concatenation of list_of_columns with sep
     """
+    if sep == '':
+        return np.sum(list_of_columns, axis=0)
     list_with_sep = [sep] * (2 * len(list_of_columns) - 1)
     list_with_sep[::2] = list_of_columns
     return np.sum(list_with_sep, axis=0)