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/ProtobufModule.h

@@ -44,6 +44,8 @@ template <class T> class ProtobufModule : protected SinglePortModule
     {
         // Update our local node info with our position (even if we don't decide to update anyone else)
         meshtastic_MeshPacket *p = allocDataPacket();
+        if (!p)
+            return nullptr;
 
         p->decoded.payload.size =
             pb_encode_to_bytes(p->decoded.payload.bytes, sizeof(p->decoded.payload.bytes), fields, &payload);