core/bloombits, eth/filters: handle null topics (#15195)

When implementing the new bloombits based filter, I've accidentally broke null
topics by removing the special casing of common.Hash{} filter rules, which
acted as the wildcard topic until now.

This PR fixes the regression, but instead of using the magic hash
common.Hash{} as the null wildcard, the PR reworks the code to handle nil
topics during parsing, converting a JSON null into nil []common.Hash topic.