KDE frequencies bug

[trvrb]

@huddlej ---

After merging #420, we're getting errors on the augur frequencies --method kde step of the seasonal flu test build visible on Travis CI: https://travis-ci.com/nextstrain/seasonal-flu/builds/140726643#L812 and reprinted below:

augur frequencies --method kde --tree results/tree_cdc_h3n2_ha_12y_cell_hi.nwk --metadata results/metadata_h3n2_ha.tsv --narrow-bandwidth 0.08333333333333333 --wide-bandwidth 0.25 --proportion-wide 0.0 --weights config/frequency_weights_by_region.json --weights-attribute region --pivot-interval 6 --min-date 2007.952772073922 --max-date 2019.870636550308 --output auspice/flu_cdc_h3n2_ha_12y_cell_hi_tip-frequencies.json

Traceback (most recent call last):
  File "/Users/trvrb/.pyenv/versions/3.6.1/bin/augur", line 11, in <module>
    load_entry_point('nextstrain-augur', 'console_scripts', 'augur')()
  File "/Users/trvrb/Documents/src/augur/augur/__main__.py", line 10, in main
    return augur.run( argv[1:] )
  File "/Users/trvrb/Documents/src/augur/augur/__init__.py", line 74, in run
    return args.__command__.run(args)
  File "/Users/trvrb/Documents/src/augur/augur/frequencies.py", line 165, in run
    frequencies = kde_frequencies.estimate(tree)
  File "/Users/trvrb/Documents/src/augur/augur/frequency_estimators.py", line 1148, in estimate
    weight_keys, weight_values = zip(*sorted(self.weights.items()))
ValueError: not enough values to unpack (expected 2, got 0)