Handle nil value in NewFactValue

[arbulu89]

Handle nil value in NewFactValue. For instance, the new saptune gatherer can have some null values, so we need to send them as null.
This doesn't mean that in many gatherers, if we get kind of null value we don't need to send a fact gathering error.

Fact output example. The () is the null value in rhai:

#{
  "$schema": "file:///usr/share/saptune/schemas/1.0/saptune_solution_verify.schema.json",
  "argv": "saptune --format json solution verify",
  "command": "solution verify",
  "exit_code": 0,
  "messages": [],
  "pid": 11179,
  "publish_time": "2023-09-26 16:02:18.474",
  "result": #{
    "attentions": [],
    "notes_enabled": [],
    "system_compliance": (),
    "verifications": []
  }
} 

How it was tested

I tested together with wanda and web.
The value arrives properly what we visualize in the frontend as NULL_VALUE. I don't know if we need to change something there:

[rtorrero]

LGTM @arbulu89, thanks!

[EMaksy]

Not an golang expert, but lgtm :D