Null types should be implicitly casted to Decimal types

scala> Seq(BigDecimal(10)).toDF("v1").selectExpr("v1 = NULL").explain(true)
org.apache.spark.sql.AnalysisException: cannot resolve '(`v1` = NULL)' due to data type mismatch: differing types in '(`v1` = NULL)' (decimal(38,18) and null).; line 1 pos 0;
'Project [(v1#5 = null) AS (v1 = NULL)#7]
+- Project [value#2 AS v1#5]
   +- LocalRelation [value#2]
...