fix: Aggregation and Union __bool__ always returning True (#1234)

The __bool__ methods were returning bool(cursor) instead of
bool(cursor.fetchone()[0]), causing them to always return True
regardless of whether the query result was empty.

Fixes #1234

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: dimitri-yatsenko

src/datajoint/expression.py

@@ -1039,7 +1039,11 @@ def __len__(self):
         ).fetchone()[0]
 
     def __bool__(self):
-        return bool(self.connection.query("SELECT EXISTS({sql})".format(sql=self.make_sql())))
+        return bool(
+            self.connection.query(
+                "SELECT EXISTS({sql})".format(sql=self.make_sql())
+            ).fetchone()[0]
+        )
 
 
 class Union(QueryExpression):
@@ -1101,7 +1105,11 @@ def __len__(self):
         ).fetchone()[0]
 
     def __bool__(self):
-        return bool(self.connection.query("SELECT EXISTS({sql})".format(sql=self.make_sql())))
+        return bool(
+            self.connection.query(
+                "SELECT EXISTS({sql})".format(sql=self.make_sql())
+            ).fetchone()[0]
+        )
 
 
 class U: