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/DropzoneModule.cpp

@@ -68,6 +68,8 @@ meshtastic_MeshPacket *DropzoneModule::sendConditions()
     // the dropzone is open
     auto dropzoneStatus = analogRead(A1) < 100 ? "OPEN" : "CLOSED";
     auto reply = allocDataPacket();
+    if (!reply)
+        return nullptr;
 
     auto node = nodeDB->getMeshNode(nodeDB->getNodeNum());
     if (sensor.hasSensor()) {