Wrong threshold value in threshold_otsu when passing a precomputed histogram

[maurosilber]

Description

When the histogram has bins with zero counts at the beginning, it returns zero as threshold.

Way to reproduce

import numpy as np
from skimage.filters import threshold_otsu

x = np.array([1, 2])

>>> threshold_otsu(x)
1
>>> threshold_otsu(hist=np.bincount(x))
0

Version information

# Paste the output of the following python commands
from __future__ import print_function
import sys; print(sys.version)
import platform; print(platform.platform())
import skimage; print("scikit-image version: {}".format(skimage.__version__))
import numpy; print("numpy version: {}".format(numpy.__version__))

3.9.6 | packaged by conda-forge | (default, Jul 11 2021, 03:39:48) [GCC 9.3.0]
Linux-5.8.0-63-generic-x86_64-with-glibc2.31
scikit-image version: 0.19.0.dev0
numpy version: 1.21.1