GDScript: Fix uninitialized local variables not being reset

modules/gdscript/gdscript_disassembler.cpp

@@ -360,6 +360,13 @@ void GDScriptFunction::disassemble(const Vector<String> &p_code_lines) const {
 
 				incr += 3;
 			} break;
+			case OPCODE_ASSIGN_NULL: {
+				text += "assign ";
+				text += DADDR(1);
+				text += " = null";
+
+				incr += 2;
+			} break;
 			case OPCODE_ASSIGN_TRUE: {
 				text += "assign ";
 				text += DADDR(1);