Tooltips/Hover over shows ??? - when missing value - how to suppress the ??? and leave it blank or '-'

Question

I am plotting geo location of some events, if there is location with only one event the second one appears as ???. How to suppress the ??? or change to '-' if there is missing value? Or if the event happens more than one time under same location, how to display once?

Here is example of the output:

And here is the graph code:

tile_provider = get_provider(CARTODBPOSITRON)
output_file('Geological_Disasters.html')
curdoc().theme = 'dark_minimal'

tooltips = [('Volcano Location', '@{Volcano Name}'),
        ('Earthquake Location', '@place'),
       ]
p = figure(plot_width=800, plot_height=600,
           x_range=(world_lon1, world_lon2), y_range=(world_lat1, world_lat2),
           x_axis_type="mercator", y_axis_type="mercator",
           tools="pan, box_select, zoom_in, zoom_out, save, reset, hover, box_zoom",
           tooltips=tooltips,
           title='Geological Disasters')

p.add_tile(tile_provider)
# 1
p.circle(x="MercatorX", y="MercatorY",size=12,fill_color="dodgerblue", 
   line_color="dodgerblue",
              source=volcano19_20)

#2
p.circle(x="MercatorX", y="MercatorY", size=3,fill_color="red", line_color="red",
     line_alpha=0.1,fill_alpha=0.1,source=earthquakes19_20)
show(p)