fix: remove some empty fields from imported yaml model

[JonathanRob]

Main improvements in this PR:

The importYaml function defines and creates a set of model fields at the start of import, which means that if some fields are missing from the .yml file, the corresponding fields in the Matlab structure will be empty. Since some functions expect fields to be of a certain length (e.g., those relating to metabolites, genes, and reactions), empty fields may cause errors in those functions.

This PR implements a change in the importYaml function that checks if any model fields are still empty at the end of the import process, and removes those fields. A few exceptions are included to avoid removing some fields which are OK to be empty (description, version).

This PR also addresses an Issue in the Mouse-GEM repository.

I hereby confirm that I have:

• Tested my code on my own computer for running the model
• Selected develop as a target branch

[haowang-bioinfo]

this PR enables importYaml to get rid of empty and non-essential fields - it looks fine to me