Werk #12764: agent_prometheus: don't crash if the cAdvisor is used without a namespace prefix

This werk only affects you if you use the scrape target cAdvisor with the option "Entity level used to create Checkmk piggyback hosts" set to "Pod". If "Prepend namespace prefix for hosts" was not selected, the agent crashed with an error message. To be consistent with the remaining entity levels and the scrape target "Kube-state-metrics" the value is now mandatory.

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