[vbaranov]

Silence credo warnings:

  Warnings - please take a look                                                                                                                                           
┃ 
┃ [W] ↗ Using `length/1` is expensive, prefer comparing against an empty list.
┃       lib/ex_eth_bls.ex:192:14 #(ExEthBls.aggregate_verify)
┃ [W] ↗ Using `length/1` is expensive, prefer comparing against an empty list.
┃       lib/ex_eth_bls.ex:173:37 #(ExEthBls.fast_aggregate_verify)
┃ [W] ↗ Using `length/1` is expensive, prefer comparing against an empty list.
┃       lib/ex_eth_bls.ex:147:69 #(ExEthBls.aggregate_signatures)
┃ [W] ↗ Using `length/1` is expensive, prefer comparing against an empty list.
┃       lib/ex_eth_bls.ex:122:72 #(ExEthBls.aggregate_public_keys)