fix(ui): Fix previewing tombstone records

[flippingbits]

Fix the previewing of tombstone records, i.e., records whose value is null, by rendering all fields as null in the grid.

[sonarcloud[bot]]

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[ChrisRousey]

Doesn't seem to be fixed. Sample dataset i used is shown like this with rpk:

{
  "topic": "test-partitions",
  "key": "{\"record_key\": 9999}",
  "value": "null",
  "timestamp": 1675968141752,
  "partition": 4,
  "offset": 11927
}

The following sample doesn't produce an error, but it isn't null, it's empty:

{
  "topic": "test-partitions",
  "key": "{\"record_key\": 9994}",
  "value": "{}",
  "timestamp": 1675968686280,
  "partition": 2,
  "offset": 9872
}

[flippingbits]

This looks more like an error that is being thrown by the deserializer. It looks like you are trying to interpret the string(!) "null" as JSON?

I was trying to fix a UI bug that occurred with tombstone records, e.g.:

{
  "topic": "test-partitions",
  "key": "{\"record_key\": 9994}",
  "value": null,
  "timestamp": 1675968686280,
  "partition": 2,
  "offset": 9872
}

I will provide a test case next time - My apologies!

[flippingbits]

Thanks for investigating and clarifying the issue and of course for the review! :)