Graphics: Fix ::transition() "filename" default value

The default value is an empty string, which triggers the simple
transition. Passing null is not legal (and wasn't possible in
mkxp from Ruby side anyway).

Fixes #108.

binding-mri/graphics-binding.cpp

@@ -48,7 +48,7 @@ RB_METHOD(graphicsTransition)
 	RB_UNUSED_PARAM;
 
 	int duration = 8;
-	const char *filename = 0;
+	const char *filename = "";
 	int vague = 40;
 
 	rb_get_args(argc, argv, "|izi", &duration, &filename, &vague RB_ARG_END);

binding-mruby/graphics-binding.cpp

@@ -45,7 +45,7 @@ MRB_FUNCTION(graphicsFreeze)
 MRB_FUNCTION(graphicsTransition)
 {
 	mrb_int duration = 8;
-	const char *filename = 0;
+	const char *filename = "";
 	mrb_int vague = 40;
 
 	mrb_get_args(mrb, "|izi", &duration, &filename, &vague);

src/graphics.cpp

@@ -716,7 +716,7 @@ void Graphics::transition(int duration,
 		return;
 
 	vague = clamp(vague, 1, 256);
-	Bitmap *transMap = filename ? new Bitmap(filename) : 0;
+	Bitmap *transMap = *filename ? new Bitmap(filename) : 0;
 
 	setBrightness(255);
 

src/graphics.h

@@ -37,7 +37,7 @@ public:
 	void update();
 	void freeze();
 	void transition(int duration = 8,
-	                const char *filename = 0,
+	                const char *filename = "",
 	                int vague = 40);
 	void frameReset();