fix(logql): Handle nil entry in blocked queries slice

[chaudum]

Summary

This PR fixes an edge case where a nil entry in the blocked queries slice leads to a nil-pointer dereference during execution.

When the blocked queries slice is generated through the tenant limits, this should never be the case.

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Note

Low Risk
Low risk: adds a defensive nil check in the blocked-query loop and a regression test; no behavior changes for valid policies.

Overview
Prevents a nil-pointer panic when the BlockedQueries slice contains nil by skipping nil entries during evaluation.

Adds a unit test covering this edge case to ensure queries execute normally when a nil policy is present.

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