Use empty string instead of None for credentials value

[jjnicola]

What:
Set empty string as value when there is an credential param without value

Why:
The string "None" was sent as preference value instead of the empty string.

How:
Run a scan with an snmp credential without community. With this patch, you should not see a result indicating failure for snmp v1|2c. Before, a result was shown, as the script checked the snmp with community "None"

Checklist:

• Tests
• CHANGELOG Entry

[codecov]

Codecov Report

Merging #335 into ospd-20.08 will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           ospd-20.08     #335   +/-   ##
===========================================
  Coverage       73.47%   73.47%           
===========================================
  Files              23       23           
  Lines            2571     2571           
===========================================
  Hits             1889     1889           
  Misses            682      682           

Impacted Files	Coverage Δ
ospd/protocol.py	88.04% <100.00%> (ø)

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