charts: Better protect ChartRange to avoid null ranges

charts: Better protect ChartRange to avoid null ranges

Also:

* Add toString methods to chart range and range map
* Remove setMinimum and setMaximum mutators (they are not used)
* Add methods to get the pattern of the timestamp formatter

Change-Id: I4ea0fd34c06af8e09194cd6bfb5d256b334d0993
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/89468
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>