Spring Add nullToEmpty convenience method to StringUtils

It allows you to replace common patterns like:

if(string == null) {
    string = "";
}
doSomethingWith(string);

or:

return string == null ? "" : string;

with a nullToEmpty(string) method call.

Comment From: bclozel

There hasn't been much demand for this, so I'm closing this PR as a result. Thanks for your contribution!