Null-check packet allocations in allocForSending and its callers (#11086)

* Null-check packet allocations in allocForSending and its callers

Sibling of 0ae44d701.

* Null-check allocDataProtobuf, allocAckNak and allocErrorResponse callers

Second tier of the same nullable contract.

* Keep telemetry sleep scheduling on allocation failure

Allocation failure now marks the telemetry invalid instead of returning
early, so power-saving SENSOR nodes still arm deep sleep.

src/modules/SerialModule.h

@@ -71,6 +71,8 @@ class SerialModuleRadio : public MeshModule
     {
         // Update our local node info with our position (even if we don't decide to update anyone else)
         meshtastic_MeshPacket *p = router->allocForSending();
+        if (!p)
+            return nullptr;
         p->decoded.portnum = ourPortNum;
 
         return p;