Update X509_get_default_cert_* stubs to return empty strings.

According to the documentation, these functions must return static strings, so NULL was not valid.

Fixes #6474.

Author: kareem-wolfssl

src/x509.c

@@ -3448,25 +3448,25 @@ int wolfSSL_X509_pubkey_digest(const WOLFSSL_X509 *x509,
     const char* wolfSSL_X509_get_default_cert_file_env(void)
     {
         WOLFSSL_STUB("X509_get_default_cert_file_env");
-        return NULL;
+        return "";
     }
 
     const char* wolfSSL_X509_get_default_cert_file(void)
     {
         WOLFSSL_STUB("X509_get_default_cert_file");
-        return NULL;
+        return "";
     }
 
     const char* wolfSSL_X509_get_default_cert_dir_env(void)
     {
         WOLFSSL_STUB("X509_get_default_cert_dir_env");
-        return NULL;
+        return "";
     }
 
     const char* wolfSSL_X509_get_default_cert_dir(void)
     {
         WOLFSSL_STUB("X509_get_default_cert_dir");
-        return NULL;
+        return "";
     }
     #endif
 

tests/api/test_ossl_x509.c

@@ -528,10 +528,10 @@ int test_wolfSSL_X509(void)
     ExpectIntEQ(X509_verify_cert(ctx), SSL_SUCCESS);
 
 #ifndef NO_WOLFSSL_STUB
-    ExpectNull(X509_get_default_cert_file_env());
-    ExpectNull(X509_get_default_cert_file());
-    ExpectNull(X509_get_default_cert_dir_env());
-    ExpectNull(X509_get_default_cert_dir());
+    ExpectStrEQ(X509_get_default_cert_file_env(), "");
+    ExpectStrEQ(X509_get_default_cert_file(), "");
+    ExpectStrEQ(X509_get_default_cert_dir_env(), "");
+    ExpectStrEQ(X509_get_default_cert_dir(), "");
 #endif
 
     ExpectNull(wolfSSL_X509_get_der(NULL, NULL));