Task: Fix nullable operator comparison on Atomic types

[NightOwl888]

The current operator comparison logic on Atomic types is broken in 3 ways:

1. We aren't declaring the atomic (reference) types as nullable.
2. We are using GetValueOrDefault() on nullable value types to do the comparison rather than properly checking for null input. We should only return true if both sides are null if either of them are. But, we are normalizing null to be the same as 0, which is not correct.
3. Atomic (reference) types are not being checked for null, so they could throw an NRE. We need to add null checking.

See: apache/lucenenet#1064