...and d3e89269 "initial checkin -
implementations for formlayer import/export - still under construction" just
forgot the '.getLength()' in

  OSL_ENSURE(0 == _rChars.trim(), ...

that is present in other, similar code.  (And the current code happend to use
the operator ==(sal_Unicode const *, OUString const &) overload that happens to
treat a null first argument like an empty string.)

Change-Id: I9d74b6ae29ca5f5f80391de50e4898add6bf6fe2