Fix eELOSSByZone expr: access before initialization

[lbonaldo]

Description

This closes #541.
The expression eELOSSByZone is accessed in co2_cap.jl without being initialized if no storage is present in the model.

What type of PR is this? (check all applicable)

• Bug Fix

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and .md files under /docs/src have been updated if necessary.
• The latest changes on the target branch have been incorporated, so that any conflicts are taken care of before merging.
• Code has been tested to ensure all functionality works as intended.
• CHANGELOG.md has been updated.
• I consent to the release of this PR's code under the GNU General Public license.

How this can be tested

Run Example_Systems/SmallNewEngland/OneZone/Run.j.

Post-approval checklist for GenX core developers

After the PR is approved

• Check that the latest changes on the target branch are incorporated, either via merge or rebase
• Remember to squash and merge if incorporating into develop

[cfe316]

Approving; not sure if you want to merge or squash it to main.

[codecov]

Codecov Report

Merging #569 (d7de540) into main (ae19f20) will increase coverage by 0.22%.
The diff coverage is 0.00%.

@@           Coverage Diff            @@
##            main    #569      +/-   ##
========================================
+ Coverage   0.07%   0.30%   +0.22%     
========================================
  Files        112     112              
  Lines       3800    3931     +131     
========================================
+ Hits           3      12       +9     
- Misses      3797    3919     +122     

Files	Coverage Δ
src/model/generate_model.jl	0.00% <0.00%> (ø)
src/model/resources/storage/storage_all.jl	0.00% <0.00%> (ø)

... and 40 files with indirect coverage changes