fix(dao): reject nil free gas query requests

[modelsbridgeaicom-ship-it]

Summary

Fixes #1014.

FreeGasAccounts and IsFreeGasAccount dereferenced nil gRPC request pointers. This adds explicit nil-request guards that return codes.InvalidArgument before either handler reads request fields or unwraps query state.

The regression calls both handlers with nil requests and verifies they return InvalidArgument instead of panicking.

Tests

Passed:

..\..\tools\go\bin\go.exe test .\x\dao\keeper -run TestFreeGasQueriesRejectNilRequests -count=1 -v
..\..\tools\go\bin\go.exe test .\x\dao\keeper -count=1
git diff --check