Mask all TOBs with word zero

With this MR, all the TOB words with value equal to zero found in the gFEX data frame are masked and not saved in the corresponding containers. This is meant to avoid an exception thrown in HLTSeeding, caused by the "invalid" TOBs with word=0. The issue was reported and discussed in ATR-25495. Please note that this is a temporary fix, and we are discussing the best approach for propagating the information of "invalid" TOBs (for all Fexes), as they will ultimately need to be monitored as well.