add coalesce to count_a and count_b before taking absolute difference so it doesn't result as null

resolves #986

Problem

When equal_rowcount is applied between 2 tables of different groupby keys, the test still passes.

Solution

The root problem was caused by coalesce at diff_count because mismatching keys could cause null diff_count. Therefore coalesce should be applied where the absolute difference is taking place

Checklist

• This code is associated with an issue which has been triaged and accepted for development.
• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the README.md (if applicable)