Werk #15415: Interface checks: Fix errors in perfometer rendering

This issue only affected network interface services. After discovering new services or after an update, the user interface initially displayed `Exception: list index out of range` in the place where the perfometer is supposed to be displayed. The log file var/log/web.log also recorded these errors:

2023-04-11 10:00:28,884 [40] [cmk.web 78582] error rendering performeter
Traceback (most recent call last):
...
IndexError: list index out of range

These errors quickly disappeared after a couple of check cycles. As of this werk, these error messages are not displayed anymore. Instead, the perfometer is rendered as soon as all necessary metrics are found.

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