Fix NullAway/Error Prone warnings in `core`

This is a follow up issue that fixes issues were obscured by `doclint` warnings that were fixed in issues #45 and #47.

`NullAway/Error Prone` reports for multiple findings in `core`:
- Address a number of `==`/`!=` comparisons that compare object references instead of using `.equals()`/`Objects.equals()`.
- Address an `instanceof` pattern match against a field already declared as that exact type, which is equivalent to a plain null check.
- Address `@Nullable` fields that are dereferenced unconditionally.
- Address `StringSplitter` using a limit of `0`, which drops a trailing empty field.

Start in the core module by running the NullAway/Error Prone reports and locating the listed reference comparisons, redundant instanceof check, nullable dereferences, and StringSplitter usage. Review the affected code and existing tests before changing it; done means the reported warnings are resolved and StringSplitter no longer drops a trailing empty field.

Written by the indexing model from the issue text.