Collection fields with empty field values prevent indexing

Problem/Motivation

When indexing a collection field, if that field does not have a value the index will error out with "The request is invalid. Details: A null value was found for the property named 'name', which has the expected type 'Collection(Edm.String)[Nullable=False]'. The expected type 'Collection(Edm.String)[Nullable=False]' does not allow null values."

After reviewing the docs, Azure Search AI expects an empty array rather than a NULL value for collection fields.

Steps to reproduce

1. Create a collection field.
2. Ensure that at least one indexable item does not have a value set.
3. Attempt to index.

Collection field is successfully indexed.

Error is encountered and indexing stops.

Proposed resolution

Move the null check down so that we check field type and return an empty array for collections.

Remaining tasks

User interface changes

API changes

Data model changes