BUG: Don't index args unless we know we can

Otherwise we get an `IndexError` on any command that didn't require
more arguments. For example running a bare `%conda` raises currently.

In addition, `%conda list` raises as there aren't enough args for
-        args = args[1:]

IPython/core/magics/packaging.py

@@ -82,8 +82,9 b' class PackagingMagics(Magics):'
 
         conda = _get_conda_executable()
         args = shlex.split(line)
-        command = args[0]
-        args = args[1:]
+        command = args[0] if len(args) > 0 else ""
+        args = args[1:] if len(args) > 1 else [""]
+
         extra_args = []
 
         # When the subprocess does not allow us to respond "yes" during the installation,