WITH REQUIRED OPTION

Problem: The user isn't allowed to alter a table to create generated
non-nullable geometry columns if there is data in the table.

Since there is no implicit default value for geometry columns, there is
a restriction requiring non-nullable geometry columns that are added to
a table containing data to have an explicitly set default value. The
generated value is not recognized as a default value.

Solution: Recognize the generated value as the default value for the
column. However, unlike columns of other types, the generation
expression for geometry columns may not return NULL unless the column is
nullable. There is no implicit conversion of NULL values.

For stored generated columns, the expression is computed during the
ALTER statement, and if any computed value is invalid, the ALTER
statement is aborted.

Adding virtual generated columns with expressions evaluating to NULL or
something that is not a well-formed geometry is allowed. The ALTER
statement will not fail. However, whenever the expression is evaluated,
the statement will fail.

Change-Id: I310f119c0bbf26b02d04dbdff35b88fb7c714977