Force all generated Ops to have default constructors

Ops simply cannot be instantiated by the OpService if they have no
default constructors, therefore it is a bug if any Op has no default
constructor.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a regression test to prevent Ops without default constructors

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>