FIX: Treat missing field maps as empty list instead of None

[tsalo]

Changes proposed in this pull request

When field maps are ignored or unavailable, the associated variable is an empty list, but most bools associated with the field maps (i.e., use_fieldwarp) assume that they're None.

• Closes Boilerplate says it used SDC transforms even with --ignore fieldmaps #1818.
• Replaces all uses of fmaps is not None with bool(fmaps) in workflows/bold/base.py.

Documentation that should be reviewed

The generated boilerplate should now have "the transforms to correct for head-motion" instead of "a single, composite transform to correct for head-motion and susceptibility distortions" when field maps are ignored or not present in the dataset.

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### Review Comments (other than those left inline with the code)

[tsalo]

Tests appear to be failing because of the bump of sdcflows to 0.1.2, where init_sdc_wf dropped the bold_meta argument.

EDIT: It looks like that's already being dealt with elsewhere.

[oesteban]

Hi @tsalo this looks good overall. Could you confirm on your end that the boilerplate is correct when no fieldmaps are present (or ignored)?

Our tests on circle have all susceptibility correction (probably that's one reason we didn't address this earlier).

Thanks a lot!

[tsalo]

Yes, when field maps are ignored the generated boilerplate now has

The BOLD time-series (including slice-timing correction when applied)
were resampled onto their original, native space by applying
the transforms to correct for head-motion.

instead of

The BOLD time-series (including slice-timing correction when applied)
were resampled onto their original, native space by applying
a single, composite transform to correct for head-motion and
susceptibility distortions.

I don't know the downstream effects in init_bold_std_trans_wf, init_ica_aroma_wf, init_bold_t1_trans_wf, but they can only fix possible bugs, right?