I suspect the BIDS format does this for consistency. Are you planning to use these images for spatial undistortion or are you hoping to concatenate them with b-weighted images?
For a series with all B=0 volumes, one can infer that the bval file would be filled with zeros. My dcm2niix will only create bval/bvec files if there are some non-zero bvalues in the series. The reason is that my software does not know your intention - without these values it is unclear that this is a diffusion sequence (e.g. it might be a spin-echo sequence used for undistortion of a gradient-echo fMRI/resting state run).
As an aside, be very cautious when setting up a B=0 series that is designed to match other series where B>0. Scanners often will do a lot of automatic optimizations that are limited by the maximum B-value. Therefore, a series that only includes B=0 images may end up different than an identical B=0 acquired with other b-values. This explains the UK Biobank Protocol PDF: the series diff_PA_MPopt_MB3_3b0_lowflip includes a few B=2000 volumes, even though the intention is to acquire a B=0 series with reversed phase encoding polarity for undistorting diff_AP_MPopt_MB3_50b1000_50b2000_8b0_lowflip.