fix(falkordb): handle empty sanitized query in build_fulltext_query

[smcdonnell7]

Summary

FalkorDriver.build_fulltext_query crashes RediSearch when every token of the input reduces to a stopword.

After line 417 filters stopwords, sanitized_query can become the empty string. The function then emits:

(@group_id:"...") ()

RediSearch rejects this with a syntax error, which surfaces as a 500 from any Graphiti.search_() call whose subject happens to be an all-stopword phrase. Real-world triggers we hit in production:

• Entity names produced by the LLM extractor like "will send the", "the one", "it is".
• Any fact text that reduces to stopwords after sanitization.

Repro

Against any running FalkorDB instance:

from graphiti_core.driver.falkordb_driver import FalkorDriver

d = FalkorDriver(host="localhost", port=6379)
q = d.build_fulltext_query("will the it", group_ids=["my_group"])
# before this patch: '(@group_id:"my_group") ()'   <-- RediSearch syntax error
# after  this patch: '(@group_id:"my_group")'
assert "()" not in q

End-to-end, graphiti.search_(query="will send the", group_ids=["x"]) returns an empty result set instead of raising.

Fix

When the sanitized query is empty after stopword filtering, return either the empty string (no group filter) or just the group filter on its own, so RediSearch receives a valid query. Keeps the upstream behaviour for every non-empty case identical.

Test plan

• Manual repro above passes with the patch
• Normal queries ("shopify migration") unchanged
• Searches against a FalkorDB-backed Graphiti no longer 500 on all-stopword entity names

[danielchalef]

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