Strip user-agent whitespaces and ignore empty user agents

[benoit74]

Fix #281

Rationale:

• userAgent deserves a special handling to remove whitespaces:
  • leading are not allowed on some websites
  • trailing are useless especially when using in conjunction with a suffix which already adds a space
• refuse empty userAgent which are bad:
• no real reason to permit it
• and it causes issues once combined with a userAgentSuffix which adds a leading space
• do not generalize to refuse empty CLI arg since it might be needed in some occasion where ones want to blank the crawler default value

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 14.17%. Comparing base (957e52c) to head (9366669).

Files	Patch %	Lines
src/zimit/zimit.py	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #310      +/-   ##
==========================================
- Coverage   14.45%   14.17%   -0.29%     
==========================================
  Files           1        1              
  Lines         249      254       +5     
  Branches       35       38       +3     
==========================================
  Hits           36       36              
- Misses        213      218       +5     

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[benoit74]

I propose to continue to not care about codecov issues for now, I've opened #311 so that this is fixed in 2.1 (minimal coverage + sufficient code structure to be able to require proper testing of next modifications)