If both tables contain zero rows results in an error in Snowflake. Ths is caused by the consequence that group by 1 returns no rows, so the resulting test contains no rows. This is solved by removing the grouping if there are no group by columns defined

[FrankTub]

In Snowflake a query like the following returns no rows.

select 1 as id_dbtutils_test_equal_rowcount,
  count(*)
from my_table
group by 1

If both tables do not contain a row the following error will be raised:

13:10:13    None is not of type 'integer'

Failed validating 'type' in schema['properties']['failures']:
    {'type': 'integer'}

On instance['failures']:
    None
13:10:13  

This PR solves that issue