[Bug] Pie charts of data which include empty strings are incorrect

[hlmore]

Description:

If you make a pie chart of a field in which some values are empty strings '', the empty-string values each appear in the pie chart as a single section with a label equal to the index of the value in the backend dataframe.

For example, using the countries data, there are four countries with an empty string in the Region field, which a pie chart of Regions shows as 66, 115, 134, and 214:

Expected behaviour:

Fields with empty strings should either be combined into one pie section with the label '' (preferred), or be omitted altogether (this is what happens in bar charts).