Implement the Symbol class to represent null

[ogaclejapan]

The Symbol class has been implemented to represent null values for DataModel<T>. With the recent addition of Optional Query, there have been instances where a QueryObject can be nullable. To address this, a fallback function has been included in the runtime package module.

• orNone
• orPending

val query1: QueryObject<String> = rememberQuery(..)
val query2: QueryObject<String>? =  rememberQueryIf(..)
Await(query1, query2.orNone()) { data1, data2 ->
    // do something with data1 and data2
}

refs: #95