Werk #16896: mbg_lantime_ng_state: ValueError (could not convert string to float: 'n/a')

Value for {{mbgLtNgNtpRefclockOffset}} may be 'n/a' instead of a float representation, resulting in {{ValueError (could not convert string to float: 'n/a')}} being thrown in the check function.

This change skips preprocessing and evaluating a value of {{'n/a'}} for {{mbgLtNgNtpRefclockOffset}}

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