🐛 fix research-and-writing links still rendering on bake even when content is invalid

[ikesau]

The research and writing block had an incorrectly implemented fallback for when linked documents were invalid.

If there's an error and we're previewing, we show a red warning block. If we're not previewing (i.e. baking), we should return null.

This was noticed because an unpublished gdoc that was being linked to in the CO₂ topic page was showing up as an empty tile.

[owidbot]

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SVG tester:

Number of differences (default views): 0 Number of differences (all views): 0

Edited: 2024-05-15 18:47:09 UTC Execution time: 1.17 seconds