[Java]List vector child value count not set correctly

[asfimport]

We are not correctly processing list vectors that could have null values. The child value count would be off there by losing data in variable width vectors.

Reporter: Praveen Kumar / @praveenbingo
Assignee: Praveen Kumar / @praveenbingo

PRs and other links:

• GitHub Pull Request #3625

_{Note: This issue was originally created as ARROW-4539. Please see the migration documentation for further details.}