fix: Mark JsPartitionedTable.getTable as nullable (#5050)

- JsPartitionedTable.getTable returns `null` if the key is not found, as the table may not exist yet.

web/client-api/src/main/java/io/deephaven/web/client/api/JsPartitionedTable.java

@@ -27,6 +27,7 @@
 import io.deephaven.web.shared.data.RangeSet;
 import jsinterop.annotations.JsIgnore;
 import jsinterop.annotations.JsMethod;
+import jsinterop.annotations.JsNullable;
 import jsinterop.annotations.JsProperty;
 import jsinterop.annotations.JsType;
 import jsinterop.base.Js;
@@ -160,12 +161,12 @@ private void handleKeys(Event update) {
     }
 
     /**
-     * Fetch the table with the given key.
+     * Fetch the table with the given key. If the key does not exist, returns `null`.
      *
      * @param key The key to fetch. An array of values for each key column, in the same order as the key columns are.
-     * @return Promise of dh.Table
+     * @return Promise of dh.Table, or `null` if the key does not exist.
      */
-    public Promise<JsTable> getTable(Object key) {
+    public Promise<@JsNullable JsTable> getTable(Object key) {
         // Wrap non-arrays in an array so we are consistent with how we track keys
         if (!JsArray.isArray(key)) {
             key = JsArray.of(key);