[Java][Flexbuffers] Add API to add nullables into the buffer. (#7521)

Fix #7517

java/com/google/flatbuffers/FlexBuffersBuilder.java

@@ -173,6 +173,21 @@ public class FlexBuffersBuilder {
         return bb;
     }
 
+    /**
+     * Insert a null value into the buffer
+     */
+    public void putNull() {
+        putNull(null);
+    }
+
+    /**
+     * Insert a null value into the buffer
+     * @param key key used to store element in map
+     */
+    public void putNull(String key) {
+        stack.add(Value.nullValue(putKey(key)));
+    }
+
     /**
      * Insert a single boolean into the buffer
      * @param val true or false
@@ -675,6 +690,10 @@ public class FlexBuffersBuilder {
             this.iValue = Long.MIN_VALUE;
         }
 
+        static Value nullValue(int key) {
+            return new Value(key, FBT_NULL, WIDTH_8, 0);
+        }
+
         static Value bool(int key, boolean b) {
             return new Value(key, FBT_BOOL, WIDTH_8, b ? 1 : 0);
         }