Add support for IS_EMPTY metadata filters to Weaviate Vector Store integration

[andrewisplinghoff]

Description

This allows for using the IS_EMPTY metadata filter also when using a Weaviate Vector Store. This filter was introduced in #15167 and is supposed to filter for objects for which a property is set to null or has an otherwise empty value. The IsNull operator in Weaviate is a good match to what IS_EMPTY is expected to do: https://weaviate.io/developers/weaviate/api/graphql/filters#by-null-state

Version Bump?

Did I bump the version in the pyproject.toml file of the package I am updating? (Except for the llama-index-core package)

• Yes
• No

Type of Change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Your pull-request will likely not be merged unless it is covered by some form of impactful unit testing.

• I added new unit tests to cover this change
• I believe this change is already covered by existing unit tests