Cask: fixes for caveats output

[EricFromCanada]

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• Casks with no caveats now return null in their JSON representation instead of "". This fixes an empty table being shown on their cask pages at https://formulae.brew.sh.
• Recommend adoptopenjdk for casks that require Java as per Endorse AdoptOpenJDK instead of Oracle OpenJDK #6040.

[MikeMcQuaid]

• This fixes an empty table being shown on their cask pages at https://formulae.brew.sh.

You could also check for an empty string in formulae.brew.sh output but I'm fine with this so: 👍 thanks again.

[EricFromCanada]

Yes, that issue cropped up because I'd assumed the cask API would return null in that case as the formula API currently does.

[MikeMcQuaid]

Gotcha. 👍 for consistency.