minor, replace nullptr checks with implicit ones

src/hb-directwrite.cc

diff --git a/src/hb-directwrite.cc b/src/hb-directwrite.cc
index efb2029..f2fce07 100644
--- a/src/hb-directwrite.cc
+++ b/src/hb-directwrite.cc
@@ -635,7 +635,7 @@
   bool isRightToLeft = HB_DIRECTION_IS_BACKWARD (buffer->props.direction);
 
   const wchar_t localeName[20] = {0};
-  if (buffer->props.language != nullptr)
+  if (buffer->props.language)
     mbstowcs ((wchar_t*) localeName,
 	      hb_language_to_string (buffer->props.language), 20);