zsh: don't complete conflicting options

[jonchang]

Before:

% brew --cache --cask --<TAB>
--build-from-source  -- Show the cache file used when building from source
--debug              -- Display any debugging information
--force-bottle       -- Show the cache file used when pouring a bottle
--formula            -- Only show cache files for formulae
--help               -- Show this message
--quiet              -- Make some output more quiet
--verbose            -- Make some output more verbose

After:

% brew --cache --cask --<TAB>
--debug    -- Display any debugging information
--help     -- Show this message
--quiet    -- Make some output more quiet
--verbose  -- Make some output more verbose

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?
• Have you successfully run brew man locally and committed any changes?

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[BrewTestBot]

Review period will end on 2021-01-28 at 03:33:14 UTC.

[MikeMcQuaid]

Suggested change

	return "" unless conflicts.presence
	return "" if conflicts.blank?

[Rylan12]

This is great, thanks, @jonchang!

[BrewTestBot]

Review period ended.

[MikeMcQuaid]

Thanks @jonchang!