render/color: fix return value type

../render/color_lcms2.c: In function ‘wlr_color_transform_init_linear_to_icc’:
    ../render/color_lcms2.c:26:24: error: incompatible types when returning type ‘_Bool’ but ‘struct wlr_color_transform *’ was expected
       26 |                 return false;
          |                        ^~~~~

render/color_lcms2.c

@@ -23,7 +23,7 @@ struct wlr_color_transform *wlr_color_transform_init_linear_to_icc(
 	cmsContext ctx = cmsCreateContext(NULL, NULL);
 	if (ctx == NULL) {
 		wlr_log(WLR_ERROR, "cmsCreateContext failed");
-		return false;
+		return NULL;
 	}
 
 	cmsSetLogErrorHandlerTHR(ctx, handle_lcms_error);