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/Telemetry/HostMetrics.cpp

@@ -128,6 +128,8 @@ bool HostMetricsModule::sendMetrics()
     // telemetry.variant.host_metrics.has_user_string ? telemetry.variant.host_metrics.user_string : "");
 
     meshtastic_MeshPacket *p = allocDataProtobuf(telemetry);
+    if (!p)
+        return false;
     p->to = NODENUM_BROADCAST;
     p->decoded.want_response = false;
     p->priority = meshtastic_MeshPacket_Priority_BACKGROUND;