Query:
SELECT g_1.StringNum AS c_0 FROM Source.SmallA AS g_1 WHERE g_1.IntKey <= 50 UNION ALL SELECT g_0.StringNum AS c_0 FROM Source.SmallB AS g_0 WHERE g_0.IntKey > 50 ORDER BY c_0

Result: as you can see, nulls are not next to each other
|StringNum|
|-24|
|<null>|
|-14|
|-13|
|-12|
|-11|
|-10|
|-9|
|-8|
|<null>|
|-6
|-5|
|...|

Expected result: this is result of simple query SELECT stringnum FROM source.smalla ORDER BY 1
|StringNum|
|-1|
|-10|
|-11|
|-12|
|-13|
|-14|
|-15|
|...|