Fix at_least_one test when group_by_columns is configured

integration_tests/data/schema_tests/data_test_at_least_one.csv

@@ -1,2 +1,3 @@
-field
-a
+field,value
+a,1
+b,

integration_tests/models/generic_tests/schema.yml

@@ -14,8 +14,12 @@ seeds:
       - name: field
         data_tests:
           - dbt_utils.at_least_one
+      - name: value
+        data_tests:
           - dbt_utils.at_least_one:
               group_by_columns: ['field']
+              error_if: "<1"
+              warn_if: "<0"
 
   - name: data_test_expression_is_true
     data_tests:

macros/generic_tests/at_least_one.sql

@@ -26,8 +26,10 @@ from (
       select
         {{ select_pruned_cols }}
       from {{ model }}
-      where {{ column_name }} is not null
-      limit 1
+      {% if group_by_columns|length() == 0 %}
+        where {{ column_name }} is not null
+        limit 1
+      {% endif %}
     )
     select
         {# In TSQL, subquery aggregate columns need aliases #}