Add support for hiredis type REDIS_REPLY_NIL in redis_raw_query

[jarrodb]

Initial attempt to address #525

[vladpaiu]

Hello,

Thanks for the patch - merged ( also fixed a small typo ).

Best Regards,
Vlad

[jayesh1017]

Hello,
Although this works, but if I query for a key that doesn't exist in redis, opensips throws out a error in logs something like this:
ERROR:core:new_avp: id: 85 s- NULL VALUE!
This is harmless as far as I tested, but just fills up the logs unnecessarily.