[SPARK-26730][SQL] Strip redundant AssertNotNull for ExpressionEncoder's serializer

## What changes were proposed in this pull request?

For types like Product, we've already add AssertNotNull when we construct serializer(see code below), so we could strip redundant AssertNotNull for those types.

```
val fieldValue = Invoke(
    AssertNotNull(inputObject, walkedTypePath), fieldName, dataTypeFor(fieldType),
    returnNullable = !fieldType.typeSymbol.asClass.isPrimitive)
```
## How was this patch tested?

Existed.

Closes apache#23651 from Ngone51/dev-strip-redundant-assertnotnull-for-ecnoder-serializer.

Authored-by: wuyi <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

Author: Ngone51

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/ScalaReflection.scala

@@ -631,8 +631,11 @@ object ScalaReflection extends ScalaReflection {
               "cannot be used as field name\n" + walkedTypePath.mkString("\n"))
           }
 
-          val fieldValue = Invoke(
-            AssertNotNull(inputObject, walkedTypePath), fieldName, dataTypeFor(fieldType),
+          // SPARK-26730 inputObject won't be null with If's guard below. And KnownNotNul
+          // is necessary here. Because for a nullable nested inputObject with struct data
+          // type, e.g. StructType(IntegerType, StringType), it will return nullable=true
+          // for IntegerType without KnownNotNull. And that's what we do not expect to.
+          val fieldValue = Invoke(KnownNotNull(inputObject), fieldName, dataTypeFor(fieldType),
             returnNullable = !fieldType.typeSymbol.asClass.isPrimitive)
           val clsName = getClassNameFromType(fieldType)
           val newPath = s"""- field (class: "$clsName", name: "$fieldName")""" +: walkedTypePath