#150: give command_names in AutoCompleter a default value

Author: dwash96

aider/io.py

@@ -154,6 +154,8 @@ def __init__(
         self.command_completions = dict()
         if commands:
             self.command_names = self.commands.get_commands()
+        else:
+            self.command_names = []
 
         for rel_fname in addable_rel_fnames:
             self.words.add(rel_fname)