Fix null pointer dereference on sap_mgmt_con_instanceproperties.rb

Has been reported by email which a null pointer dereference can happen when trying to access protectedweb_arr:

```
In
https://github.com/rapid7/metasploit-framework/blob/master/modules/auxiliary/scanner/sap/sap_mgmt_con_instanceproperties.rb

In line 240 i added (if not protectedweb_arr or):
webmethods_output << webm if not protectedweb_arr or not
protectedweb_arr.include?(webm)

The array may be null, if the sap profile parameter
service/protectedwebmethods is set to none. This will crash the
module.
```

This patch fixes it.