Fixed failed render due to returning undefined rather than null

[jmkulwik]

In the case where no inline dropdown menu should render, such as when reviewing another student's code, the InlineDropdownMenu would return undefined rather than null. This caused the React Error https://reactjs.org/docs/error-decoder.html/?invariant=152&args[]=e to be thrown which translates to the following message:

e(...): Nothing was returned from render. This usually means a return statement is missing. Or, to render nothing, return null.

This caused the parent elements to all unmount, causing a blank screen.

This was reported during a call with CSA pilot teachers.

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PR Checklist:

• Tests provide adequate coverage
• Privacy and Security impacts have been assessed
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked