set_cam_pvs to check for EigerDetector.nexpi exist

[Relm-Arrowny]

Fixes #1796

I'm not certain if this is the definitive fix, but I've added a check for the EigerDetector nexpi parameter before attempting to set it. The latest ophyd_async has made nexpi optional.

Instructions to reviewer on how to test:

1. Do thing x
2. Confirm thing y happens

Checks for reviewer

• Would the PR title make sense to a scientist on a set of release notes
• If a new device has been added does it follow the standards
• If changing the API for a pre-existing device, ensure that any beamlines using this device have updated their Bluesky plans accordingly
• Have the connection tests for the relevant beamline(s) been run via dodal connect ${BEAMLINE}

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.10%. Comparing base (f919ae5) to head (51c24cb).

Files with missing lines	Patch %	Lines
...plans/configure_arm_trigger_and_disarm_detector.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1797      +/-   ##
==========================================
- Coverage   99.12%   99.10%   -0.02%     
==========================================
  Files         282      282              
  Lines       10683    10685       +2     
==========================================
  Hits        10589    10589              
- Misses         94       96       +2     

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[Relm-Arrowny]

Just notice #1745, so probably will be fix when it merge so closing this.