fix: guard against empty query in SPARQL generator (#870)

[SAY-5]

Fixes #870.

Guard query.split()[0] so a future refactor that loosens or removes the startswith check cannot turn an empty/whitespace response into an unhandled IndexError. The new condition uses the already-split list, so the public behavior on valid queries is unchanged.

[github-actions]

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[cybermaggedon]

@SAY-5 can you please take a look at the Contributor License Agreement notice above?

[SAY-5]

Apologies for the delay. I'm not able to sign the CLA at this time, so feel free to close this if that's a blocker. The fix itself is small and the diff stands on its own if a maintainer wants to pick it up.

[cybermaggedon]

Closed as not able to complete the CLA currently. Thanks for your interest in the project!