sp.web.getStorageEntity('mytenantproperty1')
      .then((prop: StorageEntity) => {
    //How do I handle odata.null?
      return prop.value;
});

sp.web.getStorageEntity('mytenantproperty1')
      .then((prop: StorageEntity) => {

        console.log(prop);

        if(prop["odata.null"]){
          // storage entity doesnt exist

          // do something (like create entity or return empty string)
          return "";
        }

        // storage entity exists, return its value
        return prop.Value;
});