Werk #15701: ps: Fill gaps in age metrics

The Process ... Service records metrics for the oldest and the youngest process age among all matching processes and displays them in the graphs Oldest age and Youngest age.

However, whenever there was only one matching process, or if all processes were of the exact same age, no oldest/youngest age was recorded, which lead to implausible gaps in the graphs.

To mitigate this issue, the two graphs will now be continued with the single age, even if it's technically the same value.

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