[dvsim,xcelium] Avoid an OPTP2ND error if a plusarg isn't set
[dvsim,xcelium] Avoid an OPTP2ND error if a plusarg isn't set

This is needed to get e.g. the prim_present_test testbench working
with Xcelium.

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan