Fix single ! raise error.

Closes  #14267

IPython/core/interactiveshell.py

@@ -2639,7 +2639,10 b' class InteractiveShell(SingletonConfigurable):'
         """
         cmd = self.var_expand(cmd, depth=1)
         # warn if there is an IPython magic alternative.
-        main_cmd = cmd.split()[0]
+        if cmd == "":
+            main_cmd = ""
+        else:
+            main_cmd = cmd.split()[0]
         has_magic_alternatives = ("pip", "conda", "cd")
 
         if main_cmd in has_magic_alternatives: