Fixed missing handling of nodata for count aggregator with column

[jlstevens]

The count_cat aggregators were handled but count wasn't. Simple fix that shouldn't need much discussion once a corresponding unit test is added.

[jbednar]

Thanks!

[jlstevens]

The test failure for 3.6 ubuntu seems unrelated to this PR. I tried restarting the tests but the same failure reappeared:

self = <holoviews.plotting.bokeh.callbacks.PointDrawCallback object at 0x7f197416c908>
plot_id = None

    def initialize(self, plot_id=None):
        plot = self.plot
        stream = self.streams[0]
>       cds = plot.handles['source']
E       KeyError: 'source'

@philippjfr Any idea what this failure is about? Is it simply a transient that happened twice in a row?

At any rate, happy to see this PR merged now.