Assignments NPE when model answer showTo is not set

When an Assignments model answer does not have a selection for "Show to students", it causes an NPE:

caused by: java.lang.NullPointerException
at org.sakaiproject.assignment.impl.AssignmentSupplementItemServiceImpl.canViewModelAnswer(AssignmentSupplementItemServiceImpl.java:451)

because AssignmentSupplementItemServiceImpl.java has:

                                        int show = m.getShowTo();

which is an autoboxing error (assigning Integer null to int).

The column definition for this field allows NULL values:

CREATE TABLE `ASN_MA_ITEM_T` (
  `ID` bigint(20) NOT NULL,
  `ASSIGNMENT_ID` varchar(255) DEFAULT NULL,
  `TEXT` varchar(255) DEFAULT NULL,
  `SHOW_TO` int(11) DEFAULT NULL,
  PRIMARY KEY (`ID`),
  CONSTRAINT `FKDE1CE707935EEE07` FOREIGN KEY (`ID`) REFERENCES `ASN_SUP_ITEM_T` (`ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 

This should be fixed:

• In the UI, to require a selection for "show to students"
• In the schema definition, to disallow null values
• In the code, to avoid this common autoboxing error