When populating a MultimeshInstance (node), Godot would set the
new Multimesh's color and custom data format as the current node's
multimesh, which would cause a crash if node's multimesh is null.
Populate Function will now check if node has a multimesh or not, and
set the new multimesh with default (NONE) values if node's multimesh is
null.
Fixes Issue #61553