It's a bit late to the party, since NNBD migration was done a long time ago.

But it seems that the migration process broke ListView.builder's itemBuilder ability to return null;

Steps to Reproduce

We used to be able to write the following:

ListView.builder(
  // no item count
  itemBuilder: (context, index) {
    if (condition) return null;
    return Item();
  }
)

Expected results:

Before NNBD migration, this code would render Item() until condition is true, at which point the list ends (similar to as it we reached the end of itemCount)

Actual results:

The code doesn't compile as itemBuilder doesn't allow returning null anymore.

Browsing through the git blames, it seems that this was removed by mistake.
It probably didn't have a test and slipped through the review process because of the NNBD migration of IndexedItemBuilder.

This issue #58371 (comment) in particular shows that returning null was a supported behavior.

Similarly, SliverChildBuilderDelegate.build (the delegate that implements ListView.builder) still allows returning null.
So returning null is still technically supported, just not with the .builder constructor