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/mesh/Router.cpp

@@ -194,6 +194,8 @@ PacketId generatePacketId()
 meshtastic_MeshPacket *Router::allocForSending()
 {
     meshtastic_MeshPacket *p = packetPool.allocZeroed();
+    if (!p)
+        return nullptr;
 
     p->which_payload_variant = meshtastic_MeshPacket_decoded_tag; // Assume payload is decoded at start.
     p->from = nodeDB->getNodeNum();