Fix mimikatz error when password is nil

In some cases the password value that comes out of mimikatz results is `nil`, instead of an empty string. This fixes this so that if the string is `nil` is falls back to an empty string, resulting in the call to `gsub` working instead of failing.

Before:

```
meterpreter > ssp
[!] Not currently running as SYSTEM
[*] Attempting to getprivs
[!] Did not get SeDebugPrivilege
[*] Retrieving ssp credentials
ssp credentials
===============

AuthID  Package  Domain  User  Password
------  -------  ------  ----  --------

meterpreter > wdigest
[!] Not currently running as SYSTEM
[*] Attempting to getprivs
[!] Did not get SeDebugPrivilege
[*] Retrieving wdigest credentials
[-] Error running command wdigest: NoMethodError undefined method `gsub' for nil:NilClass
````

After:

```
meterpreter > wdigest
[!] Not currently running as SYSTEM
[*] Attempting to getprivs
[!] Did not get SeDebugPrivilege
[*] Retrieving wdigest credentials
wdigest credentials
===================

AuthID                                                Package  Domain           User       Password
------                                                -------  ------           ----       --------
Erreur : Impossible d'obtenir les donn�es de session  
0;459951                                              NTLM     WIN-LBTGGLV2K4E  XXXXXXXXX  OpenProcess : (0x00000005) Access is denied. n.a. (wdigest KO)
```

[FixRM #8251]