Initializing strings as null vs empty string

How would it matter if my C++ code (as shown below) has a string initialized as an empty string :

std::string myStr = "";
....some code to optionally populate 'myStr'...
if (myStr != "") {
    // do something
}

vs. no/null initialization:

std::string myStr;
....some code to optionally populate 'myStr'...
if (myStr != NULL) {
    // do something
}

Are there any best practices or gotchas around this?

There's a function empty() ready for you in std::string:

std::string a;
if(a.empty())
{
    //do stuff. You will enter this block if the string is declared like this
}

or

std::string a;
if(!a.empty())
{
    //You will not enter this block now
}
a = "42";
if(!a.empty())
{
    //And now you will enter this block.
}

There are no gotchas. The default construction of std::string is "". But you cannot compare a string to NULL. The closest you can get is to check whether the string is empty or not, using the std::string::empty method..