[DDMD] Workaround bug in dmd: common type of string literal and null does not convert to const char *

This code is not valid in D because of http://d.puremagic.com/issues/show_bug.cgi?id=9968

The common type of `? "" : null` is `string`, but as the implicit conversion from either to `const char *` is part of the expression, not the type, the resulting `CondExp` can no longer convert.  Much simpler to just add a cast!