Reference Tests and Breaking Change: Optional nullable fields are now null instead of undefined

[wilwade]

Problem

We wanted to add tests for all the tests in https://github.com/apache/parquet-testing

Discovered Bugs

• We treated nulls as undefined, but others don't
• We incorrectly processed dictionary_page_offset >= 0 instead of only > 0

Solution

• Added new test that automatically tests all files: test/reference-test/read-all.test.ts
• Fixed found bugs

with @shannonwells

Steps to Verify:

1. Run the tests
2. Comment out the bug fixes and see reference test files fail