skip lines from input file(s) with an empty list of features

[UnixJunkie]

or we crash with an obscure error

[UnixJunkie]

or output a 0.0 prediction + WARN on such lines

[UnixJunkie]

2022-04-18 15:50:06.839 FATAL: Linwrap.increment_feat_indexes: cannot parse '' in ''
Fatal error: exception End_of_file
Raised at Stdlib__scanf.scanf_bad_input in file "scanf.ml", line 1165, characters 9-16
Called from Linwrap.increment_feat_indexes.(fun) in file "src/linwrap.ml", line 227, characters 10-138
Re-raised at Linwrap.increment_feat_indexes.(fun) in file "src/linwrap.ml", line 234, characters 9-18
Called from Stdlib__list.iter in file "list.ml", line 110, characters 12-15
Called from Linwrap.increment_feat_indexes in file "src/linwrap.ml", line 226, characters 2-327
Called from Linwrap.atom_pairs_line_to_csv in file "src/linwrap.ml", line 280, characters 29-63
Called from Stdlib__list.rev_map.rmap_f in file "list.ml", line 103, characters 22-25
Called from Utls.map_on_lines_of_file in file "src/utls.ml" (inlined), line 109, characters 2-36
Called from Linwrap.pairs_to_csv in file "src/linwrap.ml", line 295, characters 4-90
Called from Linwrap.prod_predict.(fun) in file "src/linwrap.ml", line 312, characters 13-59
Called from Parany.Parmap.parfold.(fun) in file "src/parany.ml", line 322, characters 41-46
Called from Stdlib__list.fold_left in file "list.ml", line 121, characters 24-34
Called from Linwrap.prod_predict in file "src/linwrap.ml", line 304, characters 4-765
Called from Linwrap.main in file "src/linwrap.ml", line 979, characters 8-70
Called from Linwrap in file "src/linwrap.ml", line 1082, characters 9-16