fix(subscription): handle nullable receiver / notifier info

subscription-service/src/main/kotlin/com/egm/stellio/subscription/utils/DBConversionUtils.kt

@@ -15,7 +15,7 @@ fun toZonedDateTime(entry: Any?): ZonedDateTime =
 fun toNullableZonedDateTime(entry: Any?): ZonedDateTime? =
     (entry as? OffsetDateTime)?.atZoneSameInstant(ZoneOffset.UTC)
 fun <T> toList(entry: Any?): List<T> = (entry as Array<T>).toList()
-fun toJsonString(entry: Any?): String = (entry as Json).asString()
+fun toJsonString(entry: Any?): String? = (entry as? Json)?.asString()
 inline fun <reified T : Enum<T>> toEnum(entry: Any) = enumValueOf<T>(entry as String)
 inline fun <reified T : Enum<T>> toOptionalEnum(entry: Any?) =
     (entry as? String)?.let { enumValueOf<T>(it) }