Bug 18664: Make IssueSlip returns if params are not valid

Bug 18664: Make IssueSlip returns if params are not valid

Problem raised by bug 17762: IssueSlip should return if the params are
not valid.
The tests contain 2 FIXME to highlight this problem already, it's time
to fix them.

Note that, theoretically, this change may produce software error. Indeed
the caller expecting a hashref (letter) will access the "content" key.
But that should not happen.

Test plan:
Tests must return green

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>