nas: do not generate NAS encryption key if also is set to NULL

its more a cosmetic change to clean the logs. this makes
sure that a zero key is logged when NULL encryption is selected.

lib/src/common/liblte_security.cc

@@ -211,19 +211,25 @@ LIBLTE_ERROR_ENUM liblte_security_generate_k_nas(uint8*
   LIBLTE_ERROR_ENUM err = LIBLTE_ERROR_INVALID_INPUTS;
   uint8             s[7];
 
-  if (k_asme != NULL && k_nas_enc != NULL && k_nas_int != NULL) {
-    // Construct S for KNASenc
-    s[0] = 0x15;       // FC
-    s[1] = 0x01;       // P0
-    s[2] = 0x00;       // First byte of L0
-    s[3] = 0x01;       // Second byte of L0
-    s[4] = enc_alg_id; // P1
-    s[5] = 0x00;       // First byte of L1
-    s[6] = 0x01;       // Second byte of L1
-
-    // Derive KNASenc
-    sha256(k_asme, 32, s, 7, k_nas_enc, 0);
+  if (k_asme != NULL && k_nas_enc != NULL) { //{}
+    if (enc_alg_id != LIBLTE_SECURITY_CIPHERING_ALGORITHM_ID_EEA0) {
+      // Construct S for KNASenc
+      s[0] = 0x15;       // FC
+      s[1] = 0x01;       // P0
+      s[2] = 0x00;       // First byte of L0
+      s[3] = 0x01;       // Second byte of L0
+      s[4] = enc_alg_id; // P1
+      s[5] = 0x00;       // First byte of L1
+      s[6] = 0x01;       // Second byte of L1
+
+      // Derive KNASenc
+      sha256(k_asme, 32, s, 7, k_nas_enc, 0);
+    } else {
+      memset(k_nas_enc, 0, 32);
+    }
+  }
 
+  if (k_asme != NULL && k_nas_int != NULL) {
     // Construct S for KNASint
     s[0] = 0x15;       // FC
     s[1] = 0x02;       // P0