Fix caching for zsh completions

[natebosch]

Before - failing to find completions in the relevant variable would always
force getting them fresh, regardless of whether they are present in the cache
because the || would always be true.

• in _bazel_get_options use a short circuit return when the variable is not
  empty
• in _bazel_command, _bazel_help_topic, and _bazel_info_key nest the cache
  conditions inside the if block checking for an empty varible

cleanup:

• Use the same 2 space indent for continued conditions in _bazel_get_options as
  was already used in _bazel_command and the rest

[bazel-io]

Can one of the admins verify this patch?

[damienmg]

LGTM