fix: no knobs after select

[nekitk]

Fixes #59
Fixes #30

• Storybook fails to render next knobs when you pass to select options object with an empty key. Replaced empty key with --
• Choosing empty option was setting prop value to -- which is undesired (because it is truthy). We can't set it to undefined for now (addon-knobs select should be able to have undefined options. storybookjs/storybook#4487), so I replaced it with null
• Empty option in select was available even for required props. Changed it so it is added only for props that are not required.

[sami616]

This would be a great addition. As a work around I am passing an empty string as the default value.

[RoyalHunt]

Waiting this pr for a while. Any progress?

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