it is only a string comparison used to determine if the header function should be used at all.
This is code generated by the Universal Email extension.
In the extension you could leave the redirect blank. if you do, that code would be written as:
//Send Mail All Entries
if ("" !="") {
header("Location: ");
}
If the if statement where left out in this case, it would generate an error due to the missing parameter in the header function.