WPStringUtils methods cause exception if parameter is nil

[Aaron Douglas]

Update method to be safer with a nil parameter.

[Jorge Bernal]

It's common behavior in Cocoa:

NSString *foo = [NSString stringWithString:nil]; // Crash

IMO, classes should check before calling

[Aaron Douglas]

I could argue both ways since the method doesn't say it's not nil safe - I guess I'm more accustomed to thing being naturally safe with nils unless I'm initializing.