Fix error when autocompleting after flag

[EpicWink]

When running autocomplete after any command-line option with unspecified 'metavar' attribute, a traceback for an 'AttributeError' would be displayed. This is fixed for first checking if 'metavar' was defined. Fixes #5751

[hugovk]

Testing this (macOS High Sierra, Python 3.7), looks good:

$ pip install -U git:+https://github.com/EpicWink/pip.git
Collecting git:+https://github.com/EpicWink/pip.git
  Cloning https://github.com/EpicWink/pip.git to /private/var/folders/kt/j77sf4_n6fnbx6pg199rbx700000gn/T/pip-req-build-x7dyjhbk
  Installing build dependencies ... done
Building wheels for collected packages: pip
  Running setup.py bdist_wheel for pip ... done
  Stored in directory: /private/var/folders/kt/j77sf4_n6fnbx6pg199rbx700000gn/T/pip-ephem-wheel-cache-itr_i94m/wheels/27/91/c3/40af79629665bcca2d8f08052f2752b888e7361717a022696a
Successfully built pip
Installing collected packages: pip
  Found existing installation: pip 18.0
    Uninstalling pip-18.0:
      Successfully uninstalled pip-18.0
Successfully installed pip-18.1.dev0

$ pip ins<tab>tall --up<tab>grade p<tab>

That is:

• Typing pip ins and then pressing tab autocompletes to pip install

• Typing --up and then pressing tab autocompletes to pip install --upgrade

• Typing p and then presing tab doesn't autocomplete anything, and doesn't raise any errors.

[pradyunsg]

Thanks @EpicWink! This is exactly what I had in mind. :)

[pradyunsg]

@EpicWink Would you like to work on another issue?

[lock]

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