Derivation even if missing data

[remi-kazeroni]

Describe the bug
When running the recipe_preprocessor_derive_test.yml using the skip_nonexistent option, the ESMValCore tries to do the derivation on 0-len cube that correspond to missing files and this results in an error:
ERROR esmvalcore.preprocessor:274 Failed to run concatenate([], {})

On Mistral, the mising data being:

      ctotal:
        <<: *alb_cmip5
        mip: Lmon
        additional_datasets:
          - {dataset: CanESM2, ensemble: r1i1p1}

Please attach

• The recipe that you are trying to run, you can find a copy in the run directory in the output directory
• The main_log_debug.txt file, this can also be found in the run directory in the output directory
  recipe_preprocessor_derive_test.yml.txt
  main_log_debug.txt

[remi-kazeroni]

@valeriupredoi has started to look into this in #972. See his comments here and here