storybook: Docs only build not working after upgrading to 5.2.1

Describe the bug Building with --docs no longer just builds the docs tab. It has the canvas tab as well.

To Reproduce Steps to reproduce the behavior:

1. Build storybook with --docs
2. Load storybook
3. See canvas tab is loaded

Expected behavior No tabs, just defaulting to the docs

Screenshots

System:

    @storybook/addon-docs: ^5.2.1 => 5.2.1 
    @storybook/addon-links: ^5.2.1 => 5.2.1 
    @storybook/addon-options: ^5.2.1 => 5.2.1 
    @storybook/addon-storysource: ^5.2.1 => 5.2.1 
    @storybook/addons: ^5.2.1 => 5.2.1 
    @storybook/cli: ^5.2.1 => 5.2.1 
    @storybook/react: ^5.2.1 => 5.2.1 ```