[C++ SDK] Added missing default value for CaCerts in IAM providers (#19924)

Author: Gazizonoki

.github/last_commit.txt

@@ -1 +1 @@
-81ac0de27c1d3727dcc4ef3a2f2581f441e57e41
+6065f1e3f7655f65b808df9cee77081380d85c3b

include/ydb-cpp-sdk/client/iam/common/types.h

@@ -33,7 +33,7 @@ struct TIamEndpoint {
     TDuration RefreshPeriod = NIam::DEFAULT_REFRESH_PERIOD;
     TDuration RequestTimeout = NIam::DEFAULT_REQUEST_TIMEOUT;
     bool EnableSsl = NIam::DEFAULT_ENABLE_SSL;
-    std::string CaCerts;
+    std::string CaCerts = "";
 };
 
 struct TIamJwtFilename : TIamEndpoint { std::string JwtFilename; };