fix index generation for optimized hist fc with regression models

[dennisbader]

Checklist before merging this PR:

• Mentioned all issues that this PR fixes or addresses.
• Summarized the updates of this PR under Summary.
• Added an entry under Unreleased in the Changelog.

Summary

• fixes bug when performing optimized historical forecasts with RegressionModel and last_points_only=False, where the forecast index generation could result in out-of-bound dates.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.10%. Comparing base (71a1902) to head (a7cea7d).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2623      +/-   ##
==========================================
- Coverage   94.15%   94.10%   -0.06%     
==========================================
  Files         139      139              
  Lines       14992    14992              
==========================================
- Hits        14116    14108       -8     
- Misses        876      884       +8     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.