Issue 78128

This is a follow-up of issue 78098. Therein, it was requested that the report
header should be generated before the page header on the first page, since this
is what other applications do, and this is what allows to do tabular reports
including a report header.

The agreement within the iTeam was that tabular reports should not be done by
placing the table header controls in the page header, but in the header of a
artificial group, which groups by a constant value.

Trying to do this reveals that it is currently not supported by the engine -
which boils down to: You cannot do reports which look like this:

page 1:
    +-------------------------------+
    |          Report Header        |
    +-----------+---------+---------+
    |  Field 1  | Field 2 | Field 3 |
    +-----------+---------+---------+
    | data      | data    | data    |
    | data      | data    | data    |
    | data      | data    | data    |
    | data      | data    | data    |
    | data      | data    | data    |
    | data      | data    | data    |
    | data      | data    | data    |
    | data      | data    | data    |
    +-----------+---------+---------+
page 2:
    +-----------+---------+---------+
    |  Field 1  | Field 2 | Field 3 |
    +-----------+---------+---------+
    | data      | data    | data    |
    | data      | data    | data    |
    | data      | data    | data    |
    | data      | data    | data    |
    | data      | data    | data    |
    | data      | data    | data    |
    | data      | data    | data    |
    | data      | data    | data    |
    +-----------+---------+---------+

The rows "|  Field 1  | Field 2 | Field 3 |" could either be page headers -
which doesn't work because of issue 78098 - or headers of a constant group -
which doesn't work because of this issue here.

To reproduce:
- open the attached database document
- open the report "countries, tabular" for editing
- note that the report contains one group, which is set up to group by
  a constant value "1"
- execute the report (enter "Europe" when asked for a parameter)
=> you'll get an error message saying that the report could not be executed.
   The "More" button in this message box gives you an error saying
java stack trace:
com.sun.star.lang.IllegalArgumentException
	at
com.sun.star.report.pentaho.SOReportJobFactory$_SOReportJobFactory.execute(SOReportJobFactory.java:197)