Fix an issue with exclusion list not getting updated when needed

When updating the persistent cache, a sanity check ensured the
exclusion list loaded before trying to use it. However, as an empty
array was used as the default value (and the value used when the store
was reset), the sanity check never triggered.

It's unlikely this has caused issues before, as the exclusion list
should've always been updated before updating the persistent cache.
This however changed when Splash screen was changed to skip unnecessary
steps to get the user to profile selection screen faster.

src/store/modules/TsModsModule.ts

@@ -53,7 +53,7 @@ export const TsModsModule = {
         connectionError: '',
         deprecated: new Map<string, boolean>(),
         /*** Packages available through API that should be ignored by the manager */
-        exclusions: [],
+        exclusions: undefined,
         /*** Mod list is automatically and periodically updated in the background */
         isBackgroundUpdateInProgress: false,
         /*** All mods available through API for the current active game */
@@ -121,7 +121,7 @@ export const TsModsModule = {
             state.cache = new Map<string, CachedMod>();
             state.connectionError = '';
             state.deprecated = new Map<string, boolean>();
-            state.exclusions = [];
+            state.exclusions = undefined;
             state.mods = [];
             state.modsLastUpdated = undefined;
         },
@@ -248,6 +248,8 @@ export const TsModsModule = {
             {dispatch, rootState, state},
             {chunks, indexHash}: {chunks: PackageListChunks, indexHash: string}
         ) {
+            // Splash screen skipped this if there was no need to update the
+            // package list. It may be needed later by e.g. the background update.
             if (state.exclusions === undefined) {
                 await dispatch('updateExclusions');
             }