Fix LanceDB db_uri validation check

[mohiuddin-khan-shiam]

Fix LanceDB db_uri validation check

The _validate_vector_store_db_uri method compared store.db_uri.strip (the method) to an empty string, which was always False.
As a result, whitespace-only or empty db_uri values for LanceDB were silently accepted, leading to mis-configured vector-store paths and runtime failures when later resolving or connecting to the database.

This patch calls strip() before the comparison:

if not store.db_uri or store.db_uri.strip() == "":

Now the validator correctly rejects blank or whitespace-only URIs, preventing invalid configurations from slipping through and improving overall robustness of the GraphRAG setup process.