Allow fm step configuration values to be empty strings

[berland]

Issue
Resolves bug that can only be triggered through the plugin system

Approach
Detect empty string

• 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 unit tests pass locally after every commit (git rebase -i main --exec 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

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

[berland]

Changing the test to use hypothesis makes the test run for 12 seconds. Maybe it is better to skip hypothesis and just test some normal samples manually?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.85%. Comparing base (01156bb) to head (37653eb).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ert/config/ert_config.py	92.30%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9584   +/-   ##
=======================================
  Coverage   91.85%   91.85%           
=======================================
  Files         433      433           
  Lines       26768    26770    +2     
=======================================
+ Hits        24587    24589    +2     
  Misses       2181     2181           

Flag	Coverage Δ
cli-tests	39.76% <84.61%> (+<0.01%)	⬆️
everest-models-test	34.58% <84.61%> (+<0.01%)	⬆️
gui-tests	72.14% <84.61%> (+0.02%)	⬆️
integration-test	37.20% <84.61%> (+<0.01%)	⬆️
performance-tests	51.94% <84.61%> (-0.01%)	⬇️
test	40.44% <84.61%> (-0.01%)	⬇️
unit-tests	74.18% <92.30%> (+<0.01%)	⬆️

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[codspeed-hq]

CodSpeed Performance Report

Merging #9584 will degrade performances by 16.61%

_{Comparing berland:fm_config_empty (37653eb) with main (01156bb)}

Summary

❌ 1 regressions
✅ 21 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	berland:fm_config_empty	Change
❌	test_direct_dark_performance[gen_x: 20, sum_x: 20 reals: 10-summary_with_obs-get_parameters]	144 µs	172.7 µs	-16.61%