Handle missing export in locale and metadata store

In the future, removing exports is likely going to be a feature.  Handle
the case where the export data is removed from a mod pack in the locale
and prototype metadata stores.

packages/web_ui/src/store/export_locale_stores.ts

@@ -13,6 +13,14 @@ class ExportLocaleStore {
 	_emptyMap = new Map();
 
 	async loadLocale(modPackId: number, cacheEntry: ExportLocaleCacheEntry) {
+		if (!cacheEntry.localePath) {
+			cacheEntry.data = new Map();
+			for (const callback of cacheEntry.callbacks) {
+				callback();
+			}
+			return;
+		}
+
 		let response = await fetch(`${staticRoot}static/${cacheEntry.localePath}`);
 		if (response.ok) {
 			cacheEntry.data = new Map(await response.json() as ExportLocale);

packages/web_ui/src/store/export_prototype_metadata_store.ts

@@ -16,6 +16,16 @@ interface MetadataCacheEntry {
 class ExportPrototypeMetadataStore {
 	cache = new Map<number, MetadataCacheEntry>();
 	async loadMetadata(modPackId: number, cacheEntry: MetadataCacheEntry) {
+		if (!cacheEntry.metadataPath) {
+			// eslint-disable-next-line node/no-sync
+			cacheEntry.styleSheet.replaceSync("");
+			cacheEntry.metadata = new Map();
+			for (const callback of cacheEntry.callbacks) {
+				callback();
+			}
+			return;
+		}
+
 		const response = await fetch(`${staticRoot}static/${cacheEntry.metadataPath}`);
 		if (!response.ok) {
 			notify(