Minor fixes & improvements

This pull request consists of the following:

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**Remove duplicate description from number input**

I think the intended behavior is not to display the description below the numeric title, as it is displayed next to the number input already.

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**Render first tab if trying to access an undefined tab id**

When trying to access a tab that has not been defined, a blank page is displayed (plus an ugly PHP Notice). The behavior here should be exactly the same as if no tab query parameter had been specified.

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**Only show &lt;script&gt; tag when js code specified in code option**

To keep the markup clean, it's best not to add empty `<script>` tags when there is no code to be displayed in the JavaScript code option.

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Cheers