Fixed TypeError when Port of gvm.connections.SSHConnection is None

[Korkmatik]

What: If None is passed for any value in the constructor of SSHConnection, TLSConnection, UnixSocketConnection or GvmConnection then the default value will be used.

Why: TypeError will be thrown if port is None, even though Port is Optional. Closes #320

How: I've added tests

Checklist:

• Tests
• CHANGELOG Entry
• Documentation

[codecov]

Codecov Report

Merging #321 into master will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #321      +/-   ##
==========================================
+ Coverage   97.02%   97.15%   +0.12%     
==========================================
  Files          18       18              
  Lines        4005     4005              
==========================================
+ Hits         3886     3891       +5     
+ Misses        119      114       -5     

Impacted Files	Coverage Δ
gvm/connections.py	57.48% <100.00%> (+2.99%)	⬆️

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[bjoernricks]

Thanks a lot for your contributions. I really appreciate that!

Passing None and not passing a value for using a default are two different things. When I did add the typing a misunderstood the meaning of Optional. My understanding was it is optional to pass an argument. if not passed the default will be used. But actually Optional[int] means Union[int, None].

So you PR actually implemented the right meaning of Optional but the behavior for the other parameters like hostname is still different. Either passing None should always use the default value or an exception should be raised here instead.

[bjoernricks]

Just a small thing (assertIsInstance)