Allow empty field for ensemble smoother and manual

[andreas-el]

Issue
Resolves #8459

Fix the remaining panels utilising the Stringbox validation

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure tests pass locally (after every commit!)

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.66%. Comparing base (b1c5062) to head (48944ab).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8535      +/-   ##
==========================================
- Coverage   90.67%   90.66%   -0.01%     
==========================================
  Files         342      342              
  Lines       20942    20942              
==========================================
- Hits        18989    18987       -2     
- Misses       1953     1955       +2     

Flag	Coverage Δ
gui-tests	75.81% <ø> (-0.02%)	⬇️
integration-tests	54.08% <ø> (+<0.01%)	⬆️
unit-tests	68.78% <ø> (ø)

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[larsevj]

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