Dictionary filter skips null values when evaluating not-equals.

// Repeat the values to get dictionary encoding.
Seq(Some("A"), Some("A"), None).toDF.repartition(1).write.mode("overwrite").parquet("/tmp/foo")
spark.read.parquet("/tmp/foo").where("NOT (value <=> 'A')").show()
+-----+
|value|
+-----+
+-----+

// Use plain encoding.
Seq(Some("A"), None).toDF.repartition(1).write.mode("overwrite").parquet("/tmp/bar")
spark.read.parquet("/tmp/bar").where("NOT (value <=> 'A')").show()
+-----+
|value|
+-----+
| null|
+-----+