phy,enb: only initialize NR workers if needed

the NR support in the eNB isn't finished yet but loading
all DL objects even for 4G only isn't required. So only
even attempt to load them if a NR cell is configured.

srsenb/src/phy/phy.cc

@@ -130,8 +130,12 @@ int phy::init(const phy_args_t&            args,
   parse_common_config(cfg);
 
   // Add workers to workers pool and start threads
-  lte_workers.init(args, &workers_common, log_sink, WORKERS_THREAD_PRIO);
-  nr_workers.init(args, &workers_common, log_sink, WORKERS_THREAD_PRIO);
+  if (not cfg.phy_cell_cfg.empty()) {
+    lte_workers.init(args, &workers_common, log_sink, WORKERS_THREAD_PRIO);
+  }
+  if (not cfg.phy_cell_cfg_nr.empty()) {
+    nr_workers.init(args, &workers_common, log_sink, WORKERS_THREAD_PRIO);
+  }
 
   // For each carrier, initialise PRACH worker
   for (uint32_t cc = 0; cc < cfg.phy_cell_cfg.size(); cc++) {