fix clearing a skybox model when it isn't the mission default

The previous way to clear a skybox was to set it to "default", and if the mission had no skybox then it would be cleared.  The skybox code can correctly handle the lack of a skybox, but if you pass an empty string to the skybox preloader it fails with a file not found error.  This fixes the preloader, and also adds "none" as an explicit no-skybox parameter.

Author: Goober5000

code/parse/sexp.cpp

@@ -3508,7 +3508,7 @@ int check_sexp_syntax(int node, int return_type, int recursive, int *bad_node, i
 				if ( type2 != SEXP_ATOM_STRING )
 					return SEXP_CHECK_TYPE_MISMATCH;
 
-				if ( stricmp(CTEXT(node), NOX("default")) != 0 && !strstr(CTEXT(node), NOX(".pof")) )
+				if ( stricmp(CTEXT(node), NOX("default")) != 0 && stricmp(CTEXT(node), NOX("none")) != 0 && !strstr(CTEXT(node), NOX(".pof")) )
 					return SEXP_CHECK_INVALID_SKYBOX_NAME;
 
 				break;
@@ -19141,13 +19141,12 @@ void sexp_set_skybox_model_preload(const char *name)
 {
 	int i;
 
-	if ( !stricmp("default", name) ) {
-		// if there isn't a mission skybox model then don't load one
-		if ( strlen(The_mission.skybox_model) ) {
-			i = model_load( The_mission.skybox_model, 0, nullptr );
-			model_page_in_textures( i );
-		}
-	} else {
+	if (!stricmp("default", name)) {
+		name = The_mission.skybox_model;
+	}
+
+	// if there isn't a skybox model then don't load one
+	if ( strlen(name) && stricmp(name, "none") != 0 ) {
 		i = model_load( name, 0, nullptr );
 		model_page_in_textures( i );
 	}
@@ -36735,7 +36734,7 @@ SCP_vector<sexp_help_struct> Sexp_help = {
 		"\t3-8:\tSet or unset the following skyboxes flags (optional)\r\n"
 		"\t\t\tadd-lighting, no-transparency, add-zbuffer\r\n"
 		"\t\t\tadd-culling, no-glowmaps, force-clamp\r\n\r\n"
-		"Note: If the model filename is set to \"default\" with no extension then it will switch to the mission supplied default skybox."
+		"Note: If the model filename is set to \"default\" with no extension then it will switch to the mission supplied default skybox.  If it is an empty string or \"none\" then it will remove any current skybox."
 	},
 
 	// Goober5000

code/starfield/starfield.cpp

@@ -2221,7 +2221,7 @@ void stars_set_background_model(const char *model_name, const char *texture_name
 		return;
 	}
 
-	if (model_name != nullptr && *model_name != '\0') {
+	if (model_name != nullptr && *model_name != '\0' && stricmp(model_name, "none") != 0) {
 		new_model = model_load(model_name, 0, nullptr, -1);
 
 		if (texture_name != nullptr && *texture_name != '\0') {