According to the NMODL spec:

If the SUFFIX statement is absent, the file name is used as the suffix (with the addition of an underscore character).

The following file creates a mod_suffix which is an empty string:

NEURON {
}

STATE { x }

INITIAL {
  x = 42
}

BREAKPOINT {
    SOLVE dX METHOD cnexp
}

DERIVATIVE dX { x' = -x }

This can be verified by looking at the generated cpp file, which has the following functions:

void nrn_init_
void nrn_state_
static void nrn_jacob_