Werk #12500: Call parse function even for empty data

This change only affect users with own SNMP section plugins developed against the new API.

Previously the parse function of an SNMP section (or SNMP check plugin) has only been called if at least some data could be fetched from the host (the SNMP device).

In the old API (Checkmk version < 2.0) this behaviour could be changed by setting the 'handle_empty_info' key - only then would the parse function be called for empty data.

The new API will allways call the parse function, if the sections raw data is present in the fetched data (even if it is empty). It is the programmers responsibility to deal with an potentially empty data set.

This allows for a subtle distiction: If the parse function returns None, no further processing is done, it is regarded as 'not present'. An empty return value could represent some valid information, such as an empty list of error messages.

Note that this behaviour is consistent with the TCP data sources' sections, where we also may encounter sections that are present, but empty.

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