Fix compatibility with legacy external projects (#9346)

Author: snaury

ydb/core/base/tablet.h

@@ -341,6 +341,9 @@ struct TEvTablet {
     struct TEvTabletActive : public TEventLocal<TEvTabletActive, EvTabletActive> {
         TString VersionInfo;
 
+        // Compatibility with legacy external projects
+        TEvTabletActive() = default;
+
         explicit TEvTabletActive(TString&& versionInfo)
             : VersionInfo(std::move(versionInfo))
         {}