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

@@ -211,7 +211,8 @@ 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) {
+  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
@@ -223,7 +224,12 @@ LIBLTE_ERROR_ENUM liblte_security_generate_k_nas(uint8*
 
       // 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