Fixed #33114 -- Defined default output_field of StringAgg.

Thanks Simon Charette for the review.

tests/postgres_tests/models.py

@@ -160,6 +160,7 @@ class AggregateTestModel(PostgreSQLModel):
     To test postgres-specific general aggregation functions
     """
     char_field = models.CharField(max_length=30, blank=True)
+    text_field = models.TextField(blank=True)
     integer_field = models.IntegerField(null=True)
     boolean_field = models.BooleanField(null=True)
     json_field = models.JSONField(null=True)