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?