Make template optional for GEN_KW

[andreas-el]

Issue
Resolves #6196

Backports commit 978c0f6

Approach
Short description of the approach

(Screenshot of new behavior in GUI if applicable)

Pre review checklist

• Read through the code changes carefully after finishing work
• Make sure tests pass locally (after every commit!)
• Prepare changes in small commits for more convenient review (optional)
• PR title captures the intent of the changes, and is fitting for release notes.
• Updated documentation
• Ensured that unit tests are added for all new behavior (See
  Ground Rules),
  and changes to existing code have good test coverage.

Pre merge checklist

• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.

[codecov-commenter]

Codecov Report

Merging #6197 (432aebb) into version-6.0 (33178ef) will increase coverage by 0.02%.
Report is 1 commits behind head on version-6.0.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           version-6.0    #6197      +/-   ##
===============================================
+ Coverage        81.79%   81.81%   +0.02%     
===============================================
  Files              351      351              
  Lines            21843    21852       +9     
  Branches           758      758              
===============================================
+ Hits             17867    17879      +12     
+ Misses            3724     3720       -4     
- Partials           252      253       +1     

Files	Coverage Δ
src/ert/config/ensemble_config.py	97.01% <100.00%> (+0.05%)	⬆️
src/ert/config/gen_kw_config.py	98.42% <100.00%> (+0.52%)	⬆️
src/ert/config/parsing/config_schema.py	97.36% <ø> (ø)

... and 3 files with indirect coverage changes

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[andreas-el]

Closed with Won't fix due to issue being resolved in bleeding and will be part of upcoming release quite soon.