Fix division by zero in yield calibration

[DavidhoPIK]

:bird: Description of this PR :bird:

This PR fixes a division by zero error that occures when using the h16 or h16s5 region mapping using 67k preprocessing.

The value sum((cell(i,j),w), fm_croparea("y1995",j,w,knbe14)) in LN 130 of the preloop is above but very close to zero. Like this the >0 condition is fulfilled but still LN 134 results in division by zero.

:wrench: Checklist for PR creator :wrench:

• [x] Label pull request from the label list.

  • Low risk: Simple bugfixes (missing files, updated documentation, typos) or changes in start or output scripts
  • Medium risk: Uncritical changes in the model core (e.g. moderate modifications in non-default realizations)
  • High risk: Critical changes in model core or default settings (e.g. changing a model default or adjusting a core mechanic in the model)

• [x] Self-review own code

  • No hard coded numbers and cluster/country/region names.
  • The new code doesn't contain declared but unused parameters or variables.
  • magpie4 R library has been updated accordingly and backwards compatible where necessary.
  • scenario_config.csv has been updated accordingly (important if default.cfg has been updated)

• [x] Document changes

  • Add changes to CHANGELOG.md
  • Where relevant, put In-code documentation comments
  • Properly address updates in interfaces in the module documentations
  • run goxygen::goxygen() and verify the modified code is properly documented

• [x] Perform test runs

  • Low risk:
  • Run a compilation check via Rscript start.R --> "compilation check"
  • Medium risk:
  • Run test runs via Rscript start.R --> "test runs"
  • Check logs for errors/warnings
  • High risk:
  • Run test runs via Rscript start.R --> "test runs"
  • Check logs for errors/warnings
  • Default run from the PR target branch for comparison
  • Provide relevant comparison plots (land-use, emissions, food prices, land-use intensity,...)

:chart_with_downwards_trend: Performance changes :chart_with_upwards_trend:

• Current develop branch default : ** mins
• This PR's default : ** mins

:rotating_light: Checklist for reviewer :rotating_light:

• PR is labeled correctly
• Code changes look reasonable
  • No hard coded numbers and cluster/country/region names.
  • No unnecessary increase in module interfaces
  • model behavior/performance is satisfactory.
• Changes are properly documented
  • CHANGELOG is updated correctly
  • Updates in interfaces have been properly addressed in the module documentations
  • In-code documentation looks appropriate
• [ ] content review done (at least 1)
• [x] RSE review done (at least 1)