table_diff: Only show a sample for columns that actually changed

[erindru]

Given the following scenario:

• Table A has 10 columns
• Table B has 10 columns
• sqlmesh table_diff A:B --show-sample shows 8 columns are identical (100% match) and 2 columns have differences (<100% match)

When we print the sample data tables for each column, we still print 10 tables even though only 2 columns had changes.

This results in a string of empty tables that create visual noise:

We should only output sample tables for the columns that changed