fix: load plugin object style without options object

[zvinless]

Fixes issue at the end of #9559, e.g.:

module.exports = {
    plugins: [
        {
            resolve: 'gatsby-plugin-react-helmet'
        }
    ]
}

where the plugin is loaded using object syntax but no options object is defined.

This wasn't caught by the unit test because the issue was occurring after the early out when it's the ___TEST___ plugin.

[pieh]

Thanks @zvinless!

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