Fix Histogram for events without timestamp

[jkppr]

Is your feature request related to a problem? Please describe. Some timelines have events without timestamps or timestamps that default to unix timestamp 0 (1 January 1970). When opening the histogram for such a timeline events without a timestamp will show as events in 1601 for example. This makes the histogram not useful for those initial views.

Describe the solution you'd like Ignore events entries without timestamps or default timestamps in the histogram. (Could also be a setting)