[API] Fix inputText with resizable ImString crashes if the ImGuiInputTextCallback is null

fixes #158

imgui-binding/src/main/java/imgui/ImGui.java

@@ -3147,7 +3147,7 @@ public class ImGui {
         userData.textInputData = &textInputData;
         userData.env = env;
         userData.allowedChars = allowedChars;
-        userData.handler = &callback;
+        userData.handler = callback != NULL ? &callback : NULL;
 
         bool valueChanged;