tendermint: allow null in EventAttribute kv JSON (#1376)

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.

.changelog/unreleased/breaking-changes/1375-allow-null-in-event-attributes.md

@@ -0,0 +1,4 @@
+- `[tendermint]` Allow null values in `key` and `value` fields of
+  `EventAttribute` when deserializing. The serialization schema for the fields
+  is changed to `Option<String>`
+  ([\#1375](https://github.com/informalsystems/tendermint-rs/issues/1375)).

tendermint/src/abci/event.rs

@@ -1,6 +1,7 @@
 use serde::{Deserialize, Serialize};
 
 use crate::prelude::*;
+use crate::serializers;
 
 /// An event that occurred while processing a request.
 ///
@@ -119,8 +120,10 @@ where
 #[derive(Clone, PartialEq, Eq, Debug, Serialize, Deserialize, Hash)]
 pub struct EventAttribute {
     /// The event key.
+    #[serde(with = "serializers::allow_null")]
     pub key: String,
     /// The event value.
+    #[serde(with = "serializers::allow_null")]
     pub value: String,
     /// Whether Tendermint's indexer should index this event.
     ///