B-physics trigger: explicitly init AtlasExtrapolator in muComb/MuComb_FTF algorithm (ATR-26749)

This is a straightforward fix for muComb/MuComb_FTF alg from mul2IOOvlpRmSequenceCfg, needed to run Bphysics chains with --CA option.

It works fine for 'HLT_mu4_l2io_L1MU3V' chain but fails for 'HLT_2mu4_bJpsimumu_L12MU3V'

Dear @sshaw , could you please check whether this fix is fine with you?

Many thanks in advance, Vladimir