* In the per-milestone statistics, group together empty and `NULL` values. Also, show a bar for empty values when the field is optional.
* In per-milestone statistics by owner or reporter, when the value is empty, make links in the progess bar (they were missing).
* Avoid a line break between the progress bar and the following value.
* Made the layout of progress bars more resilient against browser settings (typically a minimum font size).
* In ticket queries, fixed the displayed milestone value and the milestone link for tickets without milestone, when displaying the results grouped by milestone.
* In ticket queries, removed the distinction between empty and `NULL` values.
