Remove unconditional mj_markStack in mjv_addGeoms.

Prior to commit 3f855f3, calling `mjv_updateScene` and `mjv_updateSceneFromState` on an "empty" `mjData` (one without a buffer but where all sizes are also zero) was a valid operation. The `mj_markStack` call requires free stack space, so a call to `mjv_addGeoms` on an empty `mjData` results in a stack overflow whenever `mjVIS_TENDON` is enabled.

Fixes #2305.

PiperOrigin-RevId: 713418586
Change-Id: I99b5de23fe94ba9aa86eeef0700c891976c9aecc

Author: saran-t

src/engine/engine_vis_visualize.c

@@ -1728,7 +1728,7 @@ void mjv_addGeoms(const mjModel* m, mjData* d, const mjvOption* vopt,
   // spatial tendons
   objtype = mjOBJ_TENDON;
   category = mjCAT_DYNAMIC;
-  if (vopt->flags[mjVIS_TENDON] && (category & catmask)) {
+  if (vopt->flags[mjVIS_TENDON] && (category & catmask) && m->ntendon) {
     // mark actuated tendons
     mj_markStack(d);
     int* tendon_actuated = mjSTACKALLOC(d, m->ntendon, int);