Werk #16627: kube_persistent_volume_claim: resolve KeyError crash when Volume parameters are configured

Before this update, configuring 'Volume parameters' in the PVC check plugin led to a crash. This issue arose because the plugin was not designed to process trend size-related levels, which, however, were anticipated by the general filesystem function handler shared among filesystem-related check plugins. To resolve this, the general function has been modified to bypass trend computation when a trend rule is not set, a scenario always applicable to the PVC check plugin.

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