Werk #11710: PySNMP: No longer return empty string

The SNMP Backend needs to return an empty string for walks if no result is found for one OID in the walk because Checkmk can't handle this.

But PySNMP returned an empty string even if nothing was found for the walk OID.

If no result is found for this OID the walk should be completely empty. This was not the case for PySNMP which caused some issues with certain checks.

Now we return nothing if no result was found for the walk.

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