fix spamming warnings from apiTransactionProcessor

[sstanculeanu]

Reasoning behind the pull request

• SilentEncode prints a Warn log due to empty guardian address. The proper fix should be inside SilentEncode method, a simple check if len of pkBytes is 0 to return an empty string. Will be fixed in a next release.

Proposed changes

• added a length check for guardian address to avoid SilentEncode on empty string

Testing procedure

• standard system test, call either /transaction/pool/by-sender with some txs in pool, either /transaction/pool and check for warn like: bech32PubkeyConverter.SilentEncode hex buff = error = wrong size when encoding address, expected length 32, received 0

Pre-requisites

Based on the Contributing Guidelines the PR author and the reviewers must check the following requirements are met:

• was the PR targeted to the correct branch?
• if this is a larger feature that probably needs more than one PR, is there a feat branch created?
• if this is a feat branch merging, do all satellite projects have a proper tag inside go.mod?