Guard against monitored variable with pv_name equal to None

[backmari]

Description of the changes

Add change to handle users adding an empty monitored PV list, which shows up in the database as a monitored PV with pv_name equal to None.

In the future, we may want to change how monitored PV:s are added to disallow adding a non-valid value in the table.

Check all that apply:

• updated documentation
• Source added/refactored
• Added unit tests
• Added integration tests
• (If applicable) Verified that manual tests requiring the /SNS and /HFIR filesystems pass without fail

References:

• Links to IBM EWM items: Defect 15172: [WebMon] Monitored variables not appearing on the instrument status page
• Links to related issues or pull requests:

⚠️ Manual test for the reviewer

(Instructions for testing here)

Check list for the reviewer

• best software practices
  • clearly named variables (better to be verbose in variable names)
  • code comments explaining the intent of code blocks
• All the tests are passing
• The documentation is up to date
• code comments added when explaining intent

[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.69%. Comparing base (ed0a6a7) to head (ad82ebf).

Files with missing lines	Patch %	Lines
src/webmon_app/reporting/dasmon/view_util.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             next     #288      +/-   ##
==========================================
+ Coverage   82.67%   82.69%   +0.01%     
==========================================
  Files          60       60              
  Lines        4434     4437       +3     
==========================================
+ Hits         3666     3669       +3     
  Misses        768      768              

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[darshdinger]

Tested the unit test, passes! Looks good!