Statistic Listener throws NullPointerException if the start time of a task execution is not set

[schmalzing]

Are you able replicate the bug in the latest version of MontiCore.

• I am able to replicate the bug in the latest version of MontiCore

Describe the bug

As previously reported, there is sometimes the issue that the StatisticListener does not store the start time of a task execution. If the start time is missing, calculating the execution time of the task may throw a NullPointerException.

Steps to reproduce

1. Create a dummy task
2. Call afterExecution(Task) for that task without callit beforeExecution(Task) first

Expected behaviour

First, I would expect the start time to always be present. However, as we cannot reliably assure this, I then would expect the StatisticListener to not throw a NullPointerException. Instead, skip the calculation of the task's execution time and report the missing start time in the debug log.

Minimal reproducible example

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Relevant CI workflow/pipeline

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Gradle Log

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Additional information

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