Werk #15768: siemens_plc_duration: Fix crash if duration is smaller than a second

Previously, siemens_plc_duration check would crash if duration was smaller than a second with an error:

ValueError (invalid literal for int() with base 10: '0.05000000074505806')

Now, the check doesn't crash anymore and it's possible to input thresholds smaller than a second in the "Siemens PLC Duration" monitoring rule.

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