allow null values in config (fixes #165)

terminus-core/src/services/config.service.ts

@@ -38,7 +38,7 @@ export class ConfigProxy {
                     {
                         enumerable: true,
                         configurable: false,
-                        get: () => real[key] || defaults[key],
+                        get: () => (real[key] !== undefined) ? real[key] : defaults[key],
                         set: (value) => {
                             real[key] = value
                         }

terminus-terminal/src/services/sessions.service.ts

@@ -221,6 +221,9 @@ export class SessionsService {
     }
 
     private getPersistence (): SessionPersistenceProvider {
+        if (!this.config.store.terminal.persistence) {
+            return null
+        }
         return this.persistenceProviders.find(x => x.id === this.config.store.terminal.persistence) || null
     }
 }