With generated columns, INSERT with DEFAULT inside a VALUES clause inside a UNION fails.

Repro on trunk:

create table mytab (i int generated always as (j*2), j int);
insert into mytab values (default,1) union values (default,2);

ERROR XJ001: Java exception: 'ASSERT FAILED col[0] is null: org.apache.derby.shared.common.sanity.AssertFailure'.
java.sql.SQLException: Java exception: 'ASSERT FAILED col[0] is null: org.apache.derby.shared.common.sanity.AssertFailure'.
at org.apache.derby.impl.jdbc.SQLExceptionFactory40.getSQLException(SQLExceptionFactory40.java:95)
at org.apache.derby.impl.jdbc.Util.newEmbedSQLException(Util.java:142)
at org.apache.derby.impl.jdbc.Util.javaException(Util.java:299)
at org.apache.derby.impl.jdbc.TransactionResourceImpl.wrapInSQLException(TransactionResourceImpl.java:403)
at org.apache.derby.impl.jdbc.TransactionResourceImpl.handleException(TransactionResourceImpl.java:346)
at org.apache.derby.impl.jdbc.EmbedConnection.handleException(EmbedConnection.java:2204)
at org.apache.derby.impl.jdbc.ConnectionChild.handleException(ConnectionChild.java:81)
at org.apache.derby.impl.jdbc.EmbedStatement.executeStatement(EmbedStatement.java:1323)
at org.apache.derby.impl.jdbc.EmbedStatement.execute(EmbedStatement.java:625)
at org.apache.derby.impl.jdbc.EmbedStatement.execute(EmbedStatement.java:555)
at org.apache.derby.impl.tools.ij.ij.executeImmediate(ij.java:329)
at org.apache.derby.impl.tools.ij.utilMain.doCatch(utilMain.java:521)
at org.apache.derby.impl.tools.ij.utilMain.runScriptGuts(utilMain.java:363)
at org.apache.derby.impl.tools.ij.utilMain.go(utilMain.java:261)
at org.apache.derby.impl.tools.ij.Main.go(Main.java:229)
at org.apache.derby.impl.tools.ij.Main.mainCore(Main.java:184)
at org.apache.derby.impl.tools.ij.Main.main(Main.java:75)
at org.apache.derby.tools.ij.main(ij.java:59)
Caused by: java.sql.SQLException: Java exception: 'ASSERT FAILED col[0] is null: org.apache.derby.shared.common.sanity.AssertFailure'.
at org.apache.derby.impl.jdbc.SQLExceptionFactory.getSQLException(SQLExceptionFactory.java:45)
at org.apache.derby.impl.jdbc.SQLExceptionFactory40.wrapArgsForTransportAcrossDRDA(SQLExceptionFactory40.java:119)
at org.apache.derby.impl.jdbc.SQLExceptionFactory40.getSQLException(SQLExceptionFactory40.java:70)
... 17 more
Caused by: org.apache.derby.shared.common.sanity.AssertFailure: ASSERT FAILED col[0] is null
at org.apache.derby.shared.common.sanity.SanityManager.THROWASSERT(SanityManager.java:162)
at org.apache.derby.shared.common.sanity.SanityManager.THROWASSERT(SanityManager.java:147)
at org.apache.derby.impl.store.access.sort.MergeSort.checkColumnTypes(MergeSort.java:474)
at org.apache.derby.impl.store.access.sort.MergeInserter.insert(MergeInserter.java:98)
at org.apache.derby.impl.sql.execute.SortResultSet.loadSorter(SortResultSet.java:326)
at org.apache.derby.impl.sql.execute.SortResultSet.openCore(SortResultSet.java:270)
at org.apache.derby.impl.sql.execute.InsertResultSet.open(InsertResultSet.java:415)
at org.apache.derby.impl.sql.GenericPreparedStatement.executeStmt(GenericPreparedStatement.java:436)
at org.apache.derby.impl.sql.GenericPreparedStatement.execute(GenericPreparedStatement.java:317)
at org.apache.derby.impl.jdbc.EmbedStatement.executeStatement(EmbedStatement.java:1235)