Fail fast on Table.get(undefined)

It's a common pitfall to do db.someTable.get(someVariable) without checking that someVariable isn't null.

This is a common pitfall and in the case the call to Table.get() is the initial db call that the application makes, the call stack will be totally unreadable because:

1. Table.get() will first asynchronically auto-open the database.
2. Whe DB is finally opened it will fail deep inside DBCore leaving the user with an unreadable call stack.

See #1806 (comment)

src/classes/table/table.ts

@@ -78,6 +78,7 @@ export class Table implements ITable<any, IndexableType> {
   get(keyOrCrit, cb?) {
     if (keyOrCrit && keyOrCrit.constructor === Object)
       return this.where(keyOrCrit as { [key: string]: IndexableType }).first(cb);
+    if (keyOrCrit == null) return rejection(new exceptions.Type(`Invalid argument to Table.get()`));
 
     return this._trans('readonly', (trans) => {
       return this.core.get({trans, key: keyOrCrit})