tendermint: allow null in `EventAttribute` kv JSON

[mzabaluev]

Fixes #1375

When deserializing EventAttribute values, allow null values in the key and value fields. This follows the behaviour in 0.34 RPC (where the key-value data were also base64-encoded), but technically breaks the serde schema of everything that includes ABCI event data.

• [x] Referenced an issue explaining the need for the change
• [ ] Updated all relevant documentation in docs
• [ ] Updated all code comments where relevant
• [ ] Wrote tests
• [x] Added entry in .changelog/

[mzabaluev]

The coverage failure should be fixed by #1377.