fix(node): fix crash on null field name

Closes #20

Author: ObserverOfTime

src/main/java/io/github/treesitter/jtreesitter/Node.java

@@ -313,7 +313,7 @@ public List<Node> getChildrenByFieldName(String name) {
                     "Child index %s is out of bounds".formatted(Integer.toUnsignedString(index)));
         }
         var segment = ts_node_field_name_for_child(self, index);
-        return segment == MemorySegment.NULL ? null : segment.getString(0);
+        return segment.equals(MemorySegment.NULL) ? null : segment.getString(0);
     }
 
     /**

src/test/java/io/github/treesitter/jtreesitter/NodeTest.java

@@ -215,6 +215,7 @@ void getChildrenByFieldName() {
     @Test
     void getFieldNameForChild() {
         var child = node.getChild(0).orElseThrow();
+        assertNull(child.getFieldNameForChild(0));
         assertEquals("body", child.getFieldNameForChild(2));
     }