Filters in annotations crashed when used with numerical-type
aggregations (i.e. Avg, StdDev, and Variance). This was caused as the
source expressions no not necessarily have an output_field (such as the
filter field), which lead to an AttributeError: 'WhereNode' object has
no attribute output_field.
Thanks to Chuan-Zheng Lee for the report.
Regression in c690afb873 and two following
commits.