Fix nHasDragDropPayload SIGSEGV when payload doesn't exist

[Moulberry]

Description

getDragDropPayload() functions should simply return null when the payload doesn't exist.

Currently, DearImGui will return NULL from ImGui::GetDragDropPayload() when the payload does not exist. This causes a null dereference since the nHasDragDropPayload function performs no NULL check

Type of change

• [ ] Minor changes or tweaks (quality of life stuff)
• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

[SpaiR]

A very descent PR description! Thanks for the contribution 👍 I'll make a release on the weekend.