code: def b0_threshold_empty_gradient_message(bvals, idx, b0_threshold):
      """Message about the ``b0_threshold`` value resulting in no gradient
      selection.
  
      Parameters
      ----------
      bvals : (N,) ndarray
          The b-value, or magnitude, of each gradient direction.
      idx : ndarray
          Indices of the gradients to be selected.
      b0_threshold : float
          Gradients with b-value less than or equal to `b0_threshold` are
          considered to not have diffusion weighting.
  
      Returns
      -------
      str
          Message.
      """
  
      return (
          "Filtering gradient values with a b0 threshold value "
          f"of {b0_threshold} results in no gradients being "
          f"selected for the b-values ({bvals[idx]}) corresponding "
          f"to the requested indices ({idx}). Lower the b0 threshold "
          "value."
      )
  
imports: import logging
  from warnings import warn
  import numpy
  from scipy.linalg import inv, polar
  from dipy.io import gradients
  from dipy.core.onetime import auto_attr
  from dipy.core.geometry import vector_norm, vec2vec_rotmat
  from dipy.core.sphere import disperse_charges, HemiSphere
  from dipy.utils.deprecator import deprecate_with_version
label_desc: Use image node_id 2 for calling a global function with example usage: b0_threshold_empty_gradient_message(bvals, idx, b0_threshold) and returns: str, b0_threshold, str, bvals, str, idx, str