pkg/query/flamegraph_arrow: Expose diff values, dont set to null

[metalmatze]

So far, the AppendNull has been used for the first time a row showed up. Even overwriting (adding new diff values) to the row resulted in the value being returned as null, although the underlying value was there.

Old

Notice the blue functions in the first two rows:

New

Notice that the functions in the first two rows now have the proper color:

[alwaysmeticulous]

✅ Meticulous spotted zero visual differences across 210 screens tested: view results.

_{Last updated for commit 372a295. This comment will update as new commits are pushed.}

[brancz]

Very nice find! This probably fixes the diffing issues that I've been noticing lately with the new arrow based flamegraph.