fix(rpc): add division by zero guard

[rodrodros]

Note

Low Risk
Low risk test-only change that prevents a possible division-by-zero panic when the captured block contains no transactions.

Overview
Improves rpc/utils_test.go by marking getTipAverageFromBlock as a test helper and returning 0 when the block has no transactions, preventing a division-by-zero when computing the average tip.

^{Written by Cursor Bugbot for commit 6feef13. This will update automatically on new commits. Configure here.}

[rodrodros]

Mock test is failing is due to the addition of Proof Facts and unrelated to this PR change