[flang][runtime] Clear last record in internal WRITE even if nothing … (#74528)

…was written

At the end of an internal output statement, The I/O runtime fills (the
remainder of) the current record with blanks if it's the only record or
if anything had been written to it. This turns out to be wrong in the
case of a format that ends with an explicit advance to the next record,
which needs to be cleared even if nothing has been written.

Fixes llvm-test-suite/Fortran/gfortran/regression/arrayio_1.f90.