feat: add is_empty operator for filtering vector store (#1107)

.changeset/small-oranges-develop.md

packages/llamaindex/src/storage/vectorStore/PGVectorStore.ts

...s/llamaindex/src/storage/vectorStore/SimpleVectorStore.ts

packages/llamaindex/src/storage/vectorStore/types.ts

packages/llamaindex/src/storage/vectorStore/utils.ts

...s/llamaindex/tests/vectorStores/SimpleVectorStore.test.ts