fix: skip empty metadata filter in RetrieverTool to prevent vector store error (#5976)

* fix: skip empty metadata filter in RetrieverTool to prevent vector store error

When a user adds an "Additional Metadata Filter" then removes it, the
empty filter object {} was still passed to the vector store, causing
Pinecone (and potentially others) to reject it with "You must enter a
filter object with at least one key-value pair." Now checks that the
resolved filter has at least one key before assigning it.

Fixes #4900

Signed-off-by: majiayu000 <1835304752@qq.com>

* fix: rename metadatafilter to metadataFilter for camelCase consistency

Signed-off-by: majiayu000 <1835304752@qq.com>

* Delete packages/components/nodes/tools/RetrieverTool/RetrieverTool.test.ts

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Signed-off-by: majiayu000 <1835304752@qq.com>
Co-authored-by: Henry Heng <henryheng@flowiseai.com>