Werk #15459: filestats: Fix crash when monitoring a symlink with a non-existing target

Until now, the check plugin would crash if a monitored file didn't have age and size information. This case is possible for symlinks with a non-existing target, files that vanished during agent plugin run and in case of errors.

Now, the affected files will be ignored when calculating age and size extremes and the status of the file will be shown in case of 'Single file' aggregation.

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