Fix edge case in retry (threads containing only one assistant message)

Author: codeofdusk

src/gptcmd/cli.py

@@ -452,7 +452,10 @@ def do_retry(self, arg):
         Resend up through the last non-assistant, non-sticky message to GPT.
         This command takes no arguments.
         """
-        if not self._current_thread:
+        if not self._current_thread or (
+            len(self._current_thread) == 1
+            and self._current_thread[0].role == "assistant"
+        ):
             print("Nothing to retry!")
             return
         if self._current_thread != self._detached: