web: Don't show bar-row min-width if value is zero

When a bar-row goes to actual zero, make the blue part of the
bar hidden.
web: When memory usage is zero it's unknown => '?'

Display a question mark when memory usage is zero for a running
container, and hence unknown. The system probably doesn't support
displaying memory usage.

Closes #326
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
Signed-off-by: Marius Vollmer <marius.vollmer@redhat.com>