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.

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();