fix #7393 can't open tomcat configuration without workspaces opened

When no workspaces are opened, storagePath should return undefined in
accordance with the vscode implementation(https://github.com/microsoft/vscode/blob/416bb2e8d08f123df4bfb458070fcbb9fb69a3da/src/vs/workbench/api/node/extHostStoragePaths.ts#L46-L49).

packages/plugin-ext/src/plugin/plugin-manager.ts

@@ -314,7 +314,7 @@ export class PluginManagerExtImpl implements PluginManagerExt, PluginManager {
         const subscriptions: theia.Disposable[] = [];
         const asAbsolutePath = (relativePath: string): string => join(plugin.pluginFolder, relativePath);
         const logPath = join(configStorage.hostLogPath, plugin.model.id); // todo check format
-        const storagePath = join(configStorage.hostStoragePath || '', plugin.model.id);
+        const storagePath = configStorage.hostStoragePath ? join(configStorage.hostStoragePath, plugin.model.id) : undefined;
         const globalStoragePath = join(configStorage.hostGlobalStoragePath, plugin.model.id);
         const pluginContext: theia.PluginContext = {
             extensionPath: plugin.pluginFolder,