[BUG] Division by zero in get_windrose_data

[Daveiano]

See https://groups.google.com/g/weewx-user/c/sDK4kzO4vBw/m/LGpxbEAeAQAJ

Division by zero in https://github.com/Daveiano/weewx-wdc/blob/3.x/bin/user/weewx_wdc.py#L2051

weewx[869] ERROR weewx.cheetahgenerator: Evaluation of template /home/pi/weewx-data/skins/weewx-wdc/statistics.html.tmpl failed with exception '<class 'ZeroDivisionError'>'
Aug 16 22:31:32 WeeWx5 weewx[869] ERROR weewx.cheetahgenerator: **** Ignoring template /home/pi/weewx-data/skins/weewx-wdc/statistics.html.tmpl
Aug 16 22:31:32 WeeWx5 weewx[869] ERROR weewx.cheetahgenerator: **** Reason: division by zero
Aug 16 22:31:32 WeeWx5 weewx[869] ERROR weewx.cheetahgenerator: ****  Traceback (most recent call last):
Aug 16 22:31:32 WeeWx5 weewx[869] ERROR weewx.cheetahgenerator: ****    File "/home/pi/weewx-venv/lib/python3.9/site-packages/weewx/cheetahgenerator.py", line 343, in generate
Aug 16 22:31:32 WeeWx5 weewx[869] ERROR weewx.cheetahgenerator: ****      unicode_string = compiled_template.respond()
Aug 16 22:31:32 WeeWx5 weewx[869] ERROR weewx.cheetahgenerator: ****    File "_home_pi_weewx_data_skins_weewx_wdc_statistics_html_tmpl.py", line 258, in respond
Aug 16 22:31:32 WeeWx5 weewx[869] ERROR weewx.cheetahgenerator: ****    File "/home/pi/weewx-venv/lib/python3.9/site-packages/Cheetah/Template.py", line 1708, in _handleCheetahInclude
Aug 16 22:31:32 WeeWx5 weewx[869] ERROR weewx.cheetahgenerator: ****      self._CHEETAH__cheetahIncludes[_includeID].respond(trans)
Aug 16 22:31:32 WeeWx5 weewx[869] ERROR weewx.cheetahgenerator: ****    File "_home_pi_weewx_data_skins_weewx_wdc_includes_body_alternative_inc.py", line 324, in respond
Aug 16 22:31:32 WeeWx5 weewx[869] ERROR weewx.cheetahgenerator: ****    File "/home/pi/weewx-venv/lib/python3.9/site-packages/Cheetah/Template.py", line 1708, in _handleCheetahInclude
Aug 16 22:31:32 WeeWx5 weewx[869] ERROR weewx.cheetahgenerator: ****      self._CHEETAH__cheetahIncludes[_includeID].respond(trans)
Aug 16 22:31:32 WeeWx5 weewx[869] ERROR weewx.cheetahgenerator: ****    File "_home_pi_weewx_data_skins_weewx_wdc_includes_diagram_tile_wind_rose_inc.py", line 276, in respond
Aug 16 22:31:32 WeeWx5 weewx[869] ERROR weewx.cheetahgenerator: ****    File "_home_pi_weewx_data_skins_weewx_wdc_includes_diagram_tile_wind_rose_inc.py", line 120, in __errorCatcher8
Aug 16 22:31:32 WeeWx5 weewx[869] ERROR weewx.cheetahgenerator: ****    File "<string>", line 1, in <module>
Aug 16 22:31:32 WeeWx5 weewx[869] ERROR weewx.cheetahgenerator: ****    File "/home/pi/weewx-data/bin/user/weewx_wdc.py", line 2046, in get_windrose_data
Aug 16 22:31:32 WeeWx5 weewx[869] ERROR weewx.cheetahgenerator: ****      (percent / num_of_values) * 100
Aug 16 22:31:32 WeeWx5 weewx[869] ERROR weewx.cheetahgenerator: ****  ZeroDivisionError: division by zero