Fix: return 0 in equal_rowcount when both models have 0 rows

[i-125]

resolves #973

Problem

Currently, when using the equal_rowcount test with two models that have zero rows, the test incorrectly returns NULL instead of 0. This leads to test failures, as NULL values are treated as errors in the test.

Solution

This PR modifies the equal_rowcount test to return 0 when both models being compared have zero rows, instead of returning NULL. This change ensures that the test passes as expected in scenarios where both models contain no data.

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)