SelectField: Remove empty space at the beginning

[vmilan]

Description

Shortcut: https://app.shortcut.com/cartoteam/story/343957/qa-ui-export-a-workflow

The empty option was added due to some issues with tab navigation, but is not needed anymore and is causing an unwanted blank space.

Remove the white space at the beginning, that white space looks like something failed to render.

This change also affects MultipleSelectField component.

Current UI

Fix

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit afa8e83):

https://cartodb-fb-storybook-react-dev--pr772-feature-selectfi-sks7wtnt.web.app

_{(expires Wed, 20 Sep 2023 11:03:31 GMT)}

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_{Sign: 517cc4d31d7e09cf277774e034094b67c301cd4c}

[github-actions[bot]]

Pull Request Test Coverage Report for Build 6171489216

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 71.503%

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Totals
Change from base Build 6159410494:	0.0%
Covered Lines:	2261
Relevant Lines:	2931

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💛 - Coveralls