feature: Allow gui to be None

[evvaaaa]

For supporting DiamondLightSource/FastCS#142

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Project coverage is 37.82%. Comparing base (d587a48) to head (f72a3d6).

Files with missing lines	Patch %	Lines
src/fastcs_pandablocks/__init__.py	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #11      +/-   ##
==========================================
+ Coverage   37.78%   37.82%   +0.04%     
==========================================
  Files          13       13              
  Lines         921      920       -1     
==========================================
  Hits          348      348              
+ Misses        573      572       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

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