hsfz: vehicle ident string is only present in replays with non zero len.

[antoniovazquezblanco]

Checklist:

• If you are new to Scapy: I have checked CONTRIBUTING.md (esp. section submitting-pull-requests)
• I squashed commits belonging together
• I added unit tests or explained why they are not relevant
• I executed the regression tests (using cd test && ./run_tests or tox)
• If the PR is still not finished, please create a Draft Pull Request

During initial discovery, it is common to see UDP HSFZ frames with no vehicle identification string as a way to request replies from vehiles.

Do not populate the field if length of HSFZ len is zero.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.15%. Comparing base (6c658dd) to head (69a1a99).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4757      +/-   ##
==========================================
+ Coverage   79.52%   82.15%   +2.63%     
==========================================
  Files         344      362      +18     
  Lines       87082    87357     +275     
==========================================
+ Hits        69251    71772    +2521     
+ Misses      17831    15585    -2246     

Files with missing lines	Coverage Δ
scapy/contrib/automotive/bmw/hsfz.py	61.45% <ø> (ø)

... and 289 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[polybassa]

Thanks for the PR