Legend breaks plot when plotting empty scatter glyph

[philippjfr]

When plotting an empty scatter glyph, which is often useful when streaming data, the legend breaks because it cannot determine the correct marker to render the legend items. A basic example to reproduce

from bokeh.plotting import figure

p = figure(x_range=(0, 5), y_range=(0, 3))
scatter = p.scatter([], [], legend='Some legend entry')

VM1330:130 Uncaught TypeError: _.marker_funcs[a[u]] is not a function
    at e._render (<anonymous>:130:341219)
    at e.draw_legend_for_index (<anonymous>:130:341597)
    at t.draw_legend (<anonymous>:130:394695)
    at e (<anonymous>:130:159349)
    at t._draw_legend_items (<anonymous>:130:159662)
    at t.render (<anonymous>:130:158255)
    at t._paint_levels (<anonymous>:130:371299)
    at t.paint (<anonymous>:130:370650)
    at t.repaint (<anonymous>:130:369420)
    at t.<anonymous> (<anonymous>:130:366711)

This is reproducible using bokeh 1.0.0 and bokeh 1.0.1 and likely originates in this PR #8285 generalizing the scatter glyph.

[philippjfr]

This bug is basically breaking a bunch of streaming examples we have since they all start with an empty data source. So it would be good to fix this soon. I'll try to work up a PR soon.

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