Fixes nil pointer exception in steampipe plugin. Closes #678

[binaek]

Output:

binaeksarkar@Binaeks-MacBook-Pro steampipe % steampipe plugin
Steampipe plugin management.

Plugins extend Steampipe to work with many different services and providers.
Find plugins using the public registry at https://hub.steampipe.io.

Examples:

  # Install a plugin
  steampipe plugin install aws

  # Update a plugin
  steampipe plugin update aws

  # List installed plugins
  steampipe plugin list

  # Uninstall a plugin
  steampipe plugin uninstall aws

Usage:
  steampipe plugin [command]

Available Commands:
  install     Install one or more plugins
  list        List currently installed plugins
  uninstall   Uninstall a plugin
  update      Update one or more plugins

Flags:
  -h, --help   help for plugin

Global Flags:
      --install-dir string   Path to the Config Directory (defaults to ~/.steampipe) (default "~/.steampipe")
      --workspace string     Path to the workspace (defaults to current working directory)

Use "steampipe plugin [command] --help" for more information about a command.
binaeksarkar@Binaeks-MacBook-Pro steampipe %