Improve nullability support for fields of structured type

As discussed in https://lists.apache.org/thread.html/r602ac95fff23dd1ef974fb396df7be061ab861384ec42f5c57ce0bc2%40%3Cdev.calcite.apache.org%3E I would like to change the way a type of a field of a record is derived at couple of locations. This helps frameworks such as Apache Flink to build support for nullable records with not null fields.

I suggest to change:

• SqlDotOperator#deriveType
• SqlItemOperator#inferReturnType
• AliasNamespace#validateImpl
• RexBuilder#makeFieldAccessInternal
• SqlValidatorImpl.DeriveTypeVisitor#visit(SqlIdentifier)