return NULL -> return CString() for clarity and efficiency

stepmania/src/RageDisplay_D3D.cpp

@@ -391,7 +391,7 @@ CString SetD3DParams( bool &bNewDeviceOut )
 		}
 	}
 
-	return NULL;
+	return CString();
 }
 
 /* If the given parameters have failed, try to lower them. */
@@ -604,7 +604,7 @@ CString RageDisplay_D3D::TryVideoMode( VideoModeParams p, bool &bNewDeviceOut )
 	/* Palettes were lost by Reset(), so mark them unloaded. */
 	g_TexResourceToPaletteIndex.clear();
 
-	return NULL;	// mode change successful
+	return CString();	// mode change successful
 }
 
 void RageDisplay_D3D::ResolutionChanged()