Remove NULL check for set nonce

Also remove tests which would pass NULL to this function.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/library/psa_crypto_aead.c b/library/psa_crypto_aead.c
index aa266ea..033dc82 100644
--- a/library/psa_crypto_aead.c
+++ b/library/psa_crypto_aead.c
@@ -443,7 +443,7 @@
     psa_status_t status = PSA_ERROR_CORRUPTION_DETECTED;
 
     if( mbedtls_aead_check_nonce_length( operation, nonce_length )
-        != PSA_SUCCESS || nonce == NULL )
+        != PSA_SUCCESS )
     {
         return( PSA_ERROR_INVALID_ARGUMENT );
     }