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.
I would argue that the check for protectedweb_arr being nil should be done higher, say around line 234.  If there are no protected methods, then as written this conditional+loop won't be relevant (it would just print "#{rhost}:#{rport} [SAP] Unprotected Webmethods :::" but then no methods.

But, furthermore, the logic in this conditional+loop is such that it only reports methods as unprotected if there are *any* protected methods and this method is not one of them.  I'd make the argument that if there are no protected methods, then all methods are unprotected.