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

@@ -1,104 +1,105
 """Implementation of packaging-related magic functions.
 """
 #-----------------------------------------------------------------------------
 #  Copyright (c) 2018 The IPython Development Team.
 #
 #  Distributed under the terms of the Modified BSD License.
 #
 #  The full license is in the file COPYING.txt, distributed with this software.
 #-----------------------------------------------------------------------------
 
 import re
 import shlex
 import sys
 from pathlib import Path
 
 from IPython.core.magic import Magics, magics_class, line_magic
 
 
 def _is_conda_environment():
     """Return True if the current Python executable is in a conda env"""
     # TODO: does this need to change on windows?
     return Path(sys.prefix, "conda-meta", "history").exists()
 
 
 def _get_conda_executable():
     """Find the path to the conda executable"""
     # Check if there is a conda executable in the same directory as the Python executable.
     # This is the case within conda's root environment.
     conda = Path(sys.executable).parent / "conda"
     if conda.is_file():
         return str(conda)
 
     # Otherwise, attempt to extract the executable from conda history.
     # This applies in any conda environment.
     history = Path(sys.prefix, "conda-meta", "history").read_text()
     match = re.search(
         r"^#\s*cmd:\s*(?P<command>.*conda)\s[create|install]",
         history,
         flags=re.MULTILINE,
     )
     if match:
         return match.groupdict()["command"]
 
     # Fallback: assume conda is available on the system path.
     return "conda"
 
 
 CONDA_COMMANDS_REQUIRING_PREFIX = {
     'install', 'list', 'remove', 'uninstall', 'update', 'upgrade',
 }
 CONDA_COMMANDS_REQUIRING_YES = {
     'install', 'remove', 'uninstall', 'update', 'upgrade',
 }
 CONDA_ENV_FLAGS = {'-p', '--prefix', '-n', '--name'}
 CONDA_YES_FLAGS = {'-y', '--y'}
 
 
 @magics_class
 class PackagingMagics(Magics):
     """Magics related to packaging & installation"""
 
     @line_magic
     def pip(self, line):
         """Run the pip package manager within the current kernel.
 
         Usage:
           %pip install [pkgs]
         """
         self.shell.system(' '.join([sys.executable, '-m', 'pip', line]))
         print("Note: you may need to restart the kernel to use updated packages.")
 
     @line_magic
     def conda(self, line):
         """Run the conda package manager within the current kernel.
 
         Usage:
           %conda install [pkgs]
         """
         if not _is_conda_environment():
             raise ValueError("The python kernel does not appear to be a conda environment.  "
                              "Please use ``%pip install`` instead.")
 
         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,
         # we need to insert --yes in the argument list for some commands
         stdin_disabled = getattr(self.shell, 'kernel', None) is not None
         needs_yes = command in CONDA_COMMANDS_REQUIRING_YES
         has_yes = set(args).intersection(CONDA_YES_FLAGS)
         if stdin_disabled and needs_yes and not has_yes:
             extra_args.append("--yes")
 
         # Add --prefix to point conda installation to the current environment
         needs_prefix = command in CONDA_COMMANDS_REQUIRING_PREFIX
         has_prefix = set(args).intersection(CONDA_ENV_FLAGS)
         if needs_prefix and not has_prefix:
             extra_args.extend(["--prefix", sys.prefix])
 
         self.shell.system(' '.join([conda, command] + extra_args + args))
         print("\nNote: you may need to restart the kernel to use updated packages.")