fix: handling of empty password in SSHDriver and NetworkService

[flxzt]

Description
SSHDriver and NetworkService currently does not differentiate between no password set and password set to an empty string. Both cases resulted in that the SSH option -o PasswordAuthentication=no is passed.
There are cases where a password should be used, but it is set to an empty string.
This merge request fixes this.

Checklist

• PR has been tested

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.2%. Comparing base (6952f14) to head (9dcf884).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
labgrid/driver/sshdriver.py	77.7%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1814     +/-   ##
========================================
- Coverage    45.2%   45.2%   -0.1%     
========================================
  Files         173     174      +1     
  Lines       13721   13746     +25     
========================================
+ Hits         6203    6214     +11     
- Misses       7518    7532     +14     

Flag	Coverage Δ
3.10	45.2% <80.0%> (-0.1%)	⬇️
3.11	45.2% <80.0%> (-0.1%)	⬇️
3.12	45.2% <80.0%> (-0.1%)	⬇️
3.13	45.1% <80.0%> (-0.1%)	⬇️
3.14	45.1% <80.0%> (-0.1%)	⬇️
3.9	45.2% <80.0%> (-0.1%)	⬇️

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